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When determining the proper size of a vessel there are many factors to consider. Consideration must be given to the available space, the size of all openings through which the vessel must pass, the tilt up height for vertical vessels, product expansion if heated, clearance for the turbulence generated in mix vessels, the proper geometry for mix vessels, and planned future expansion. Over-sizing a vessel can lead to problems if not taken into account in the design. Vessel manufacturers may have preferred geometry’s and diameters that are the most economical for them to fabricate. When planning a vessel, it advisable to consult with the vessel manufacturer to get the most economical design.
Most states require ASME (American Society of Mechanical Engineers) certification for pressure vessels rated over 15 psig. The ASME pressure vessel code was adopted the prevent serious injury from pressure vessel failures. Vessel manufacturers must be certified to construct pressure vessels and are periodically reviewed for adherence to the code. The pressure vessel code encompasses all phases of pressure vessel design, construction, inspection, and repair. Authorized Inspectors are required to inspect and certify a vessel’s design and fabrication. The section of the code most commonly seen on pressure vessels is Section VIII Division 1, this covers unfired boilers and pressure vessels.
The shell or straight wall is the circular vertical portion of the vessel. The vessel may be of a single shell construction, it may have insulation and cladding, or a heat transfer jacket with insulation and cladding. For all vessels, the thickness of the shell is based on the pressure the shell will see (including hydrostatic), plus any additional loads created by the supports, nozzles, mixers, or other appurtenances. The ASME code has calculations which govern the thickness of all the tank components under internal and external pressure. In these thickness calculations, a joint efficiency factor appears which is dependent on the level of weld inspection. A vessel which has the welds Radiograph inspected would allow thinner shells and heads than a vessel not inspected with that technology. The cost associated with this inspection usually outweighs the lighter material cost. The thickness required by these calculations are based on the minimum thickness, the thinning of the material during forming, the metal removed by polishing, and any allowances for corrosion must also be taken into consideration. For shells under external pressure, such as vacuum vessels or open jacketed vessels where the coincident pressure is external to the shell, the stresses are much higher because buckling becomes a factor. On most vessels rated for both internal and external pressure, vacuum is usually the governing thickness, thus a small vessel rated for full vacuum may, for no additional cost, be rated for a higher internal pressure. Vacuum support rings attached to the shell are often used to allow the use of lighter material and may be more cost effective than a thicker shell. So far, it has been assumed the shape of the vessel is circular in cross section. This is the most economical shape to manufacture. Rectangular or square vessels are typically more expensive to fabricate and must be made of thicker materials as the flat shape has little inherent strength.
The ends of a tank or vessel are referred to as the heads. Heads may be of many shapes such as flat, flat and sloped for drainage, conical, toriconical (radiused), hemispherical, torispherical (radiused spherical), reverse dished, or elliptical. The most common head type is the torispherical shape. For sanitary service, it is important that the bottom selected provides full drainage of the vessel.
Flat and Flat Sloping
Flat and Flat sloping bottoms are easiest to fabricate, but a flat head has the lowest strength of any head. Flat bottoms are usually radiused for complete drainage. This radius is usually referred to as the knuckle radius. For sanitary service, a knuckle radius of at least 3/8" is recommended. Flat bottom vessels may be supported by resting on a concrete pad, or the may be supported by legs or other supports. Because of the low inherent strength of a flat bottom, some type of stiffening for the bottom may be required. A sloped bottom is usually pitched from 3/8" per ft to ¾’ per ft for easy drainage. Flat tops suffer from low strength and puddling issues. Flat tops which have many fittings or large openings are subject to severe warpage from the heat of welding. For pressure vessels, flat heads will be fairly thick.
Conical and Toriconical
Conical and toriconical heads possess more inherent strength than flat heads. Conical heads are easier to fabricate than toriconical heads, but are generally considered less sanitary. Conical bottoms, depending on the angle of the cone, provide excellent bottom drainage. For pressure vessels, toriconical heads are preferred. The ASME code limits conical heads without a knuckle radius to a maximum included half angle of 30 degrees. Manufacturers specify the conical angle differently, some specify the included angle, others the angle of the cone in reference to the tangent line of the head. The larger the included angle of the head, the weaker the head.
Torispherical heads possess high strength at a higher manufacturing cost. These heads are sometimes referred to as flanged and dished heads after the manufacturing process. The geometry of the head is composed of the crown or dish radius, the major portion of the head. The knuckle radius is the radius to the outer edge. Often a head will have a straight flange of between ½" to 2" to facilitate welding and polishing. The standard ratios of torispherical heads are 100-6 and 80-10. These numbers designate the ratio of the diameter of the head to the crown and knuckle radii of the head. A 100-6 head has a crown radius of 100% (or equal to) the diameter of the head with a knuckle radius of 6% of the diameter of the head. Thus a 72" Diameter 100-6 head has a crown (or dish) radius of 72" (1 x 72) and a knuckle radius of 4.32" (0.06 x 72). A 72" diameter 80-10 head has a crown radius of 57.6" (0.8 x 72) and a knuckle radius of 7.2" (0.1 x 72). 100-6 heads are commonly referred to as ASME heads. The larger the knuckle radius, the higher the strength of the head (allowing thinner material). The deeper the head, the more difficult it is to form. Flanged and dished heads provide good drainage.
Hemispherical have the highest strength of any head type but are also the most costly to manufacture. Because of the strength of the head, hemispherical heads are sometimes stacked together to form a heat transfer jacket. Hemispherical bottom tanks are sometimes referred to as kettles, and are commonly supplied with scraped surface agitators.
Elliptical heads are similar to torispherical heads. Elliptical heads are specified by the ration of the depth to the diameter. A 24" diameter 2:1 elliptical head is 6" deep. 2:1 Elliptical heads are the most common and can be approximated as a 90-17 torispherical head.
Reverse Dished Head
Reversed dished heads are not recommended for pressure, but are sometimes used for atmospheric applications.
Legs are the most common way to support a vessel. When designing any tank support, all the potential loads must be considered. These loads include: the weight of the vessel and contents, seismic loads, wind loads if outside, and any external piping or agitation loads. Leg bracing is an economical way to reduce the bending stresses caused by seismic or other external loads. The stresses imparted to the shell or bottom head caused by the leg loads must also be considered. Many vessel manufacturers will add a reinforcing pad at the leg attachment to reduce these stresses. The foot of the leg may be adjustable to compensate for irregularities and pitch in the floor, or may be fixed. Vessels in seismic areas, or outdoors where wind is a consideration, may need to be bolted to the ground to prevent the vessel from overturning. The use of balled feet, where permissible, allows for easier cleaning underneath the foot. For sanitary service, the threads of adjustable legs are often shrouded and sometimes sealed.
Vessels may be supported by means of lugs attached to the sidewall and bolted to an external frame. The bending stresses created by the eccentric loading of the lug may be quite high and require the use of a reinforcing pad or stiffening rings wrapped around the shell above and below the lugs.
Pad Mounted Vessels
It is common in many industries for vessels to be mounted on a poured concrete pad. The pad can be sloped to facilitate drainage, or the vessel manufacturer may provide a base which levels the slope of the bottom. For wind and seismic loads, these vessels are often secured to the pad by epoxy anchors. A steel ring may be anchored to the concrete and the vessel base welded to the steel ring.
Skirt Supported Vessels
Vessels may be supported with the use of a steel skirt attached to the sidewall. The skirt is usually anchored to a concrete pad like a pad mounted vessel. An access door in the skirt is a common feature to gain access to the outlet of the vessel.
For portability, vessels are constructed with legs and casters for portability. Vessels may also be built on a portable cart with casters. To prevent tipping, the legs supports for portable vessels are often extended out from the shell. If a vessel is to be sterilized in an autoclave, the casters must be designed for this service. Wheel brakes and swivel locks for swivel casters are often specified for ease of use. Floor jacks can lift the vessel off the caster and provide a secure footing. Where vessels are to be transported by a fork lift truck, fork lift pockets are added to the legs for this service. Vessels can be designed to be vertically stacked and easily transported.
For weighing applications, the vessel can be supported on load cells. Load cells typically have a strain gauge transducer that senses the weight of the tank and it’s contents. Some load cells are sensitive to side loads and can give inaccurate readings if there is any deflection in the tank supports. Legs for load cell service should be sufficiently braced to prevent any side loads. The piping to and from the vessel cannot be rigid, or the load cell will not give accurate results. Typically, 3 or more load cells are used to compensate for any irregularities in the liquid level or vibration, such as from mixing. The signals from all of the load cells are summed and averaged to give accurate readings. Stray welding currents can damage the sensitive strain gage sensing elements. It is recommended to remove the load cells when welding to the vessel, "dummy" load cells are sometimes available for this purpose
Most vessels require a means of access into the vessel. This access is required for fabrication, as well as maintenance. Open top vessels or vessels with hinged covers obviously don’t require a manway Smaller closed vessels may be fabricated with a large fitting or handhole to allow for the welding and finishing of the vessel. Vessels can be fabricated with the entire top hinged (even pressure vessels). On larger hinged top vessels, hydraulic or pneumatic cylinders can be used to raise and lower the heavy lid. Larger closed vessels require a manway for fabrication. A very important consideration when working with closed vessels is the inherent danger of a confined space. There may be residual harmful vapors in a vessel as well as the danger from welding. Argon is a colorless, odorless gas used in many welding processes, it is heavier than air. If welding is being performed in a vessel, the argon gas will settle in the bottom of the vessel and push out the oxygen. Asphyxiation may result under these circumstances. When working in a confined space, the proper safety precautions must be followed.
The manway may be installed in the top of the vessel, in the sidewall, and sometimes in the bottom. Sidewall manways are often obround and have a door which swings in or in and out. Top manways are typically round and can be designed for pressure service. Circular pressure manways can also be installed in the sidewall. Typical sizes of manways range from 16" to 24" in diameter. Pressure manways are sometimes fitted with spring assist mechanisms for ease of operation.
Venting and Relief Devices
All atmospheric vessels must be properly vented to prevent collapse. Atmospheric vessels are not designed for any vacuum condition, and on a large vessel a small amount of vacuum can create irreparable damage. Removable screened vents are often specified for sanitary service. The sizing of the vent is usually related to the filling or draining rate of the vessel. Sudden temperature changes can also create a vacuum condition. On large atmospheric vessels, it is often necessary to have the manway door open during CIP to prevent collapse. When filters are used as vents on atmospheric vessels, the pressure drop across the filter must be sufficiently low to prevent collapse. Diaphragm type level sensors can give inaccurate readings if the pressure drop across the filter is appreciable, differential pressure instruments are recommended. Relief devices can be specified for atmospheric vessels to prevent collapse. Sanitary rupture discs and pressure/relief valves are available.
For pressure vessels, the ASME code requires a pressure relief device. The relief device must be ASME certified for the service the device will be subjected to. Sanitary rupture discs should non-fragmenting and are available with sanitary fitting connections, such as Tri-Clamp connections. The exhaust side of the rupture disc is piped to either a downleg on the tank, or to a suitable exhaust line. The direction of flow should be clearly marked on the disc as well as the pressure, temperature, and service for which it was designed. Sanitary pressure relief valves are available, but ASME certification should be verified.
Outlets and Drains
All vessels for sanitary service must drain completely. The drain connection is typically a sanitary ferrule. The length of the dead leg, or stagnant portion in the fitting should be minimized. Flush bottom ball and diaphragm valves, as well as flush bottom rising stem plug valves are available and should be considered in applications where dead legs must be minimized. A variety of actuators and options are available for these types of valves.
Inlets and Instrument Connections
There are a wide variety of sanitary fittings for use with vessels. To minimize dead legs, the fittings should be installed as close as possible to the head and sidewall. With fittings installed on the sidewall on vessels with insulation and cladding, a sloped setback or alcove can be installed to allow the fitting to be installed close to the sidewall. There are many flush mount instrument fittings that can be used for instrumentation. Because they are flush with the surface of the vessel, they can be used on the bottom, sidewall, and tops of vessels. They are useful where the sidewall or bottoms of vessels are being scraped by a scraped surface agitator. Sanitary projectile wells are commonly used for temperature probes. There are many types of wells and instruments that are not compatible, purchasing the instrument through the vessel manufacturer eliminates any potential fit-up problems. External piping and heavy instruments and accessories connected to any portion of a vessel must be properly supported to prevent high stresses or failures at the weld connection to the vessel.
Heat Transfer Surface
For vessels which need to be heated or cooled, there are several types of heat transfer surfaces which can be applied to sidewall or bottom. The amount of heat transfer can be knowing the media temperature, starting and desired ending temperature, quantity and size of insulation, the type of product in the vessel, and the desired times. Typical heating/cooling medias which can be used in a vessel heat transfer jacket are: steam, hot and cold water, glycol, heat transfer oils, refrigerants, or any pumpable fluid. The heat transfer jacketing can be applied in distinct zones to permit the use of different heat transfer media, and to allow for variable batch sizes. The heat transfer jacket may need to be ASME certified like a vessel, especially for steam service over 15 psi. For liquid media, the lowest connection is typically the inlet and outlet the uppermost. This forces the entrained air out of the system. When liquid media is used, at least 7 gpm flow should be supplied to each jacket zone, however, higher flow rates may be necessary depending on your desired duty. For steam service, the steam is introduced into the uppermost connection, and the condensate pulled off of the bottom. The velocity of the steam supplied through the jacket depends on the type of jacketing selected.
A mechanical dimple jacket is a sheet of metal which has a uniform array of depressions or dimples pressed into the metal. Each of the dimples typically has a center hole which is fillet welded to the base metal. The dimpled sheet is formed to the contour of the shell or head, and the edges are welded around the circumference of the jacket. The thickness and pitch of the dimple layout determine the pressure rating. Depending on the application, the flow path of the media can be routed for optimal efficiency, or to clear manways, fittings or other discontinuities. Mechanical dimpled jackets typically give high pressure ratings, have a low to moderate pressure drop, and are moderate in cost.
Inflated RSW Dimple
An inflated RSW jacket is manufactured by resistance spot welding an array of spots on a thin sheet of metal to the thicker base metal. The edges are welded solid and all other forming operations are performed. The jacket is inflated, under high pressure, until the thin jacket material deforms to form a pattern of dimples. Like mechanical dimples, the flow path can be altered as needed. Inflated RSW dimple jackets have moderate pressure ratings, a high to moderate pressure drop, and low cost.
Pipe Coil or Channel Jackets
Pipe or channel jackets are simply a half pipe, or formed channel welded to the base metal. The jackets can be installed both in the flat and after forming. Pipe coil or channel jackets typically have moderate to high pressure ratings, low pressure drops and moderate to high cost.
Insulation and cladding
For vessels with heat transfer jackets, especially for steam service, it is recommended to insulate and clad or sheath the insulation. The temperature rating of the insulation must be appropriate for the service it will see. Because of the low resistance of stainless steels to chloride induced stress corrosion cracking, all insulation used must have little to no chloride content. Other considerations include the R value (or thermal resistance), and the thickness necessary for the application. Typical insulating materials used in vessel manufacturing are: polystyrene, basalt mineral wool, fiberglass, cork, polyurethane, polyisocyanurate, and foamed glass. For added protection, there are several commercial coatings which can be applied to the exterior of a vessel to provide a chloride barrier. The cladding attached to sanitary vessels are designed so there are no ledges where puddles can form. Penetrations through the cladding, such as heat transfer pipes, and fittings may fully welded, caulked or gasketed. For fully welded penetrations, consideration must be given to thermal expansion of the vessel with respect to the cladding.
Vessels designed for sanitary service are almost always cleaned in place and/or sterilized in place by a combination of heat, and the circulation of chemicals over all of the surfaces to be cleaned. Crevices, pockets, threads, and non-draining surfaces must be avoided. There are numerous types of sprayballs and spray heads designed for the cleaning of the interior surfaces of vessels. There are many different cleaning cycles used in various industries, but most cleaning cycles involve a pre-rinse of clean water, a heated caustic wash cycle, a post rinse cycle, and some type of sanitizing cycle.
The most common type of cleaning device is the sprayball, a hollow ball with an array of holes which sprays the interior of the vessel. The holes may be designed to spray up only, spray down, spray in a 360 degree pattern, or they may be custom located to hit specific areas. There are 2 types of cleaning actions utilized, there is a cascading action of a large flow of chemicals running over the surfaces, and an impingement action caused by the direct spray. The best type of action depends on the surfaces to be cleaned. Both actions can occur simultaneously. The use of more than one spray device may be required to completely clean all the surfaces. Agitator shafts, baffles, and other common protrusions in the vessel can shadow certain areas in the vessel. The undersides of mixer blades are also a potential cleaning problem. Nozzles and manway openings must also be considered. Sprayballs which have custom drilled holes should have a means of positively locating the sprayball to the correct orientation. Sprayballs are designed and drilled to operate at specific flows and pressures. Too low a supply pressure may result in the spray not hitting the surface with sufficient velocity for cleaning (if it hits at all). Too high a pressure can atomize the spray and lead to inefficient cleaning. Sprayballs can be designed and drilled for a given flow rate and pressure within reason. Some sprayballs are split to allow cleaning of any plugged holes. For critical applications, the flow, temperature, and pressure to a sprayball should be monitored to ensure there are no blockages. Rotary devices often do a very good job of impingement cleaning, but cannot be validated unless the rotary action can be proven.
Caustic chemicals are hazardous and should be treated with respect. Containing the chemicals in the vessel during CIP can be overlooked. Hinged lids without gasketing or a drip lip may not fully contain CIP solutions. Another consideration during CIP is the temperature variation. Large atmospheric vessels with side entry manways are often designed to be CIP’ed with the door open to provide proper venting for the tank. The temperature changes during a CIP cycle can create a vacuum condition. Rapid temperature fluctuations in a vessel can create dangerously high stresses. Try drinking a cup of hot coffee then bite an ice cube to understand the effect. A temperature change of no greater than 10 degrees a minute is recommended.
Mixers are commonly used in sanitary vessels to aid in heat transfer, mix ingredients, or alter the product in other ways, such as gas dispersion or emulsification. Agitation is a separate discussion, however the geometry of a tank designed for mixing is important. The tank manufacturer must be aware of the entire process the customer desires to specify the correct mixer. The rheology of the fluid is important, as well as the desired mixing levels, utilities available, time constraints, and any unusual environmental considerations such as explosive atmospheres, foaming, burn on, etc. The power draw of any mixing impeller is proportional to the fluid properties, the impeller diameter to the 5th power, and the speed cubed. Many fluids exhibit viscosity changes with temperature, this must be taken into consideration. Before entering any vessel with an agitation system, proper lockout-tagout rules must be followed. Pressure vessels may require a mechanical seal for proper operation. Mixer seals are available that allow for the large shaft deflections and runout conditions mixers are subject to. It is generally inadvisable to run mixers right at the impeller level. For most impellers, it is recommended that there be at least 1 impeller diameter of fluid above and below the impeller for proper operation.
A mixer can be best understood as a inefficient pump. The 2 main parameters that need to be known to properly size a pump are flow and pressure. Like a pump, mixers generate flow and pressure, but the pressure is generally referred to as shear. Different processes require varying amounts of flow and shear, simple fluid mixing is usually a flow dependent operation while emulsification is more shear dependent. Several processes require different amounts of flow and shear at different times in the process, multiple mixers are specified to address varying flow and shear requirements. For sanitary service, the mixer is often designed to be CIP cleaned and may be of welded, non-removable construction.
Cleaning and Passivation
Most sanitary vessel manufacturers are very careful about segregating stainless steel from carbon steel and iron to avoid free iron contamination on the surface. It is a fact that every piece of commercially available stainless steel will have been worked over with carbon steel equipment at some point prior to purchasing by the vessel manufacturer. For improved corrosion resistance, the free iron left on the surface must be removed. Passivation is a process whereby the free iron is removed and a beneficial oxidation layer or film is formed on the surface of the metal. Passivation is a combination of both a cleaning process and the forming of a protective "passive" layer. For passivation to take place, it is important that the surface of the metal is absolutely clean and free of all grease, oil, and any other contamination that may inhibit the passivation process. Passivation is performed by subjecting the metal surface to a acid to both clean the metal and form the passive layer. The exact temperature, time, concentration and type of acid vary from manufacturer to manufacturer. A typical passivation procedure is to circulate a 20-50% nitric acid solution at between 120 and 160 deg F for between ½ to 2 hours. Other acids such as citric and Phosphoric have been used for passivation treatments, but the best results are obtained from Nitric.
There are several tests to determine if the passivation has been successful. A common test is the Copper Sulfate test, in which the surface is soaked for 6 minutes with a Copper Sulfate solution, then rinsed and examined. Any free iron on the surface will show up as a copper or pink color. Salt spray and saline tests are sometimes used as a test for passivation, though not recommended by several vessel manufacturers.
Electropolishing is the electrolytic removal of metal in a highly ionic solution by means of a electric potential and current. In layman’s terms it is reverse electroplating. Electropolishing improves the surface finish of the metal by smoothing out (or removing) the high spots on the surface. Up to a 50% improvement in the average roughness height can be accomplished by electropolishing. Another advantage of electropolishing is the chemical interactions that occur on the surface of the metal. Electropolishing levels the grain boundaries of the metal, this removes sites for chemicals, dirt and microorganisms, another benefit is the reduction of surface area of the grain boundaries. Because electropolishing is performed in a chemically aggressive environment, any invisible defects in the material before electropolishing will appear after electropolishing. Furthermore, the metal removal is selective, free iron is readily removed from the surface whereas Chromium, Nickel and carbon are not. It is theorized that the Nickel, Chromium and Carbon actually form a Nichrome composition at the surface, Nichrome has excellent corrosion resistant properties. The free Chromium forms Chromium oxides that makes up the passive layer. The electropolishing process performs chemical passivation at least as good with other methods.
Thank you Alaskan Beer for supplying the following OSHA information:
To answer your most basic question, the easiest way we found to be compliant with OSHA's confined space requirements was to stop entering confined spaces totally. Obviously this can't always be helped, but in the case of our beer tanks we bought a portable cleaner made by Toftejorg (now Alfa Laval owned) that we could stick in through the manway door of the tank to clean the tanks better than the supplied tank sprayball was doing. This prevented us from having to climb in and scrub the tanks and made the OSHA inspector happy.
You could also just upgrade the type of sprayball you have inside the tank instead of buying a portable unit. Don't forget your brew vessels too. We had one inspector that only was worried about tanks for years, but when we changed to a new inspector he started telling us our brew kettles counted as confined space as well and we had to adjust our program to fit. As a simple explanation, they consider anything as confined space if it is an area where you can't enter or exit easily and there is a chance for harm to the employee from various sources such as gas, water, steam, etc.
So at some point in your future, you'll more than likely have to set up a confined space program anyway. We were able to get by for many years without entering tanks and avoiding the OSHA program, but as we expanded and bought more equipment (and got new inspectors!) we ended up with more confined spaces that we couldn't work around not entering, so we finally went ahead and put in a full program.
To get started you will need an air tester and a ventilation fan. We bought our own air tester; it has to measure carbon monoxide, explosive gases, and oxygen levels. These are pretty standard testers and will run you about $2,000 for a good one; I would highly recommend a tester that has an air pump so you can keep the tester out where it is dry while monitoring the air inside the tank. Also note that the air tester will need recalibrated yearly which is another little kit that you have to buy. Since you are in the real world and have access to equipment rental companies, you might just be able to rent one of these whenever you need to enter a confined space.
Then for the ventilation fan, we bought a cheap fire ventilation fan (part 1983K1), see www.mcmaster.com page 642 and the duct is on page 643. These fans have a back cowling that the ducting will fit into for storage and they are easily portable and move a lot of air. They are small enough to hang in front of a tank manway door as well.
You may need other equipment based on your state laws, so I'd check with the local OSHA office if they have a consultation department (not enforcement); they were really helpful to us when we set up our system. The other big issue you have to look at is whether you can easily get a person out of a confined space if they pass out or get hurt. In the case of our tanks, when someone goes in we have to put a body harness on them with a rope that a person stands outside of the tank holding in case we need to retrieve them. You are also required to make sure the second person is always outside keeping watch. And unless you can lock out the chance for gas entry into the tank you must also monitor the air inside the confined space the entire time, so having the air tester is critical for entry. With any confined space program, OSHA is very strict on having proper paperwork in place before you even consider going into a confined space, so you'll need to set up a confined space form and keep those on file. I have a sample form that we worked up that works for our state that I can email you if you like, so just let me know.
This is a brief nutshell of what you'd need to do. There are lots of options or requirements based on the different types of confined spaces that you might have onsite, so read up on it a lot and it could save you some money and hassle from getting into it further than you need. OSHA's web site has some really good info (www.osha.gov) as well.
- Alaskan Brewing Company