Circulation Systems for Single and Multiple Seal Arrangements (Part Two)

Part One Part Three

Plan 12

Plan 12 is similar to a Plan 11. The flush is taken off of the pump discharge, or and intermediate stage in the case of multiple stage pumps, through a strainer or filter to remove solids, and then through an orifice to control flow, before being introduced into the seal chamber.

API 682/ISO 221049 does not recommend this plan, as problems can arise if not closely monitored where the strainer can become clogged, the flush flow is lost, and the seal is damaged due to overheating. This can be avoided by using a differential pressure indicator or flow indicator to alert the user of impending problems. Some strainers utilize magnets or magnetic strainers to attract metallic particles like magnitite that will be present in most water systems.

Advantages

• No product contamination from an external source.
• Relatively simple piping plan.
• No reprocessing of product.
• Solids are removed from flush stream keeping the seal chamber clean.

Disadvantages

• If the product in the pump is not a good face lubricant, or has extremely low or high viscosity, the seal can become damaged depending upon the seal face material combination.
• Flush is recirculated.
• Strainer or filter will plug over time.

Plan 13

Plan 13 is a off-shoot of a Plan 11, where the flow comes out of the seal chamber and goes back to the pump suction. This also helps the seal to vent gas out of the seal chamber.

Typically, this plan is used on vertical pumps where the seal chamber is subject to pump discharge pressure at the top of the pump. However, this plan can be used on horizontal pumps, depending upon the type of impeller used and the differential pressures between seal chamber pressure and pump suction pressure. It is useful on high differential pressure applications where the use of a Plan 11 would require the use of multiple orifices.

On the other hand, for small or low speed pumps that have a low differential pressure, no orifice is required. Due to the flow path, Plan 13 is not as effective as a Plan 11 in removing seal generated heat. The path is such that it comes from the back of the seal and rises up to the gland plate outlet port with no direct impingement on the seal faces. In some cases, flow diverters may be incorporated to improve the flow path or flush rates can be increased to make up for the decrease in efficiency.

Advantages

• No product contamination from an external source.
• Relatively simple piping plan.
• No reprocessing of product.
• Continuous venting of the seal chamber.

Disadvantages

• If the product in the pump is not a good face lubricant, or has extremely low or high viscosity, the seal can become damaged depending upon the seal face material combination.
• Flush is recirculated.
• Less efficient flow pattern.

Plan 14

Plan 14 is a combination of a Plan 11 and a Plan 13. The flush is taken off of the pump discharge and sent to the seal chamber like a Plan 11. A second set of piping takes the flush from the seal chamber, sending it back to pump suction like a Plan 13.

It is often used in vertical pumps to provide adequate flush flow and vapor pressure margin independent of the throat bushing below the seal chamber. It is used on viscous products to provide a flow path out of the box in addition to the throat bushing that can be restrictive. It is also an effective plan when the throat bushing that partially controls flow rates is inadequate for the seals' needs.

Advantages

• No product contamination.
• No reprocessing of product.
• Optimized cooling. The flush flow can be controlled so that the cooling is directed at the faces and adequate flow is maintained.
• Allows complete automatic venting provided that the "FO" port in the gland is properly located.
• With a properly sized orifice and throat bushing, the seal chamber pressure remains high, resulting in an adequate vapor pressure margin.

Disadvantages

• If the product in the pump is not a good face lubricant or is dirty, the seal can become damaged or clogged.
• Flush is recirculated.

Sizing

For the above three plans, the flush rate should be calculated based on the pumping conditions to maximize efficiency and seal life. For applications above 3600-rpm or box pressures above 500-psig, the flush rate should be calculated to avoid excessive temperature at the seal faces. In lower pressure/speed applications, a "rule of thumb" of 1-gpm per inch size can be used.

Controlling

The flow rate is controlled by an orifice or series of orifices in the flush line. API 682/ISO 21049 states that the orifices should not be less than 1/8-in. The orifices do not all have to be the same size and can be larger if there is a possibility that the flush stream may clog smaller orifice sizes.

Another method is to use a "choke tube." This is a piece of tubing generally ¼-in heavy wall. The length of the tubing is calculated using a piping pressure drop calculation such that the pressure drop across the tubing is equal to the difference between the discharge or suction pressure (depending upon the specific plan) and the seal chamber pressure at the flow rate desired.

Plan 21/22

Plan 21 is again an off-shoot of a Plan 11, where the product is taken from pump discharge and directed through an orifice to a heat exchanger to reduce the flush temperature before being introduced into the seal chamber. Plan 22 is the same as a Plan 21, with the addition of a strainer located before the orifice.

A temperature indicator should be included on the process side of the exchanger, normally on the downstream side. Additional temperature indicators are used in some installations to monitor process and cooling water temperatures on both sides of the heat exchanger.

Plans 21 and 22 are not preferred plans by the API and by many users. This is due to the high heat load that is placed on the heat exchanger in these plans. For example, taking 3-gpm water from 350-deg F down to 160-deg F consumes 270,600-Btu/hr. These high heat loads result in wasted energy and high rates of fouling of the heat exchanger on both the process and cooling water side, which often results in shortened seal life.

Fouling normally occurs on the water side of cooling tower based cooling systems. This can be averted to some degree by maintaining a velocity that is sufficient to resist the deposit of cooling water sediment on the coils.

The use of a throat bushing will reduce the effects of heat soak into the seal chamber and can also increase the seal chamber pressure slightly assisting in increasing the vapor pressure margin. The standard pump fixed throat bushing may be utilized on lower temperature applications where the process fluid is not volatile.

A floating throat bushing will reduce the clearance by more than half over a fixed bushing and is recommended on applications involving higher temperatures and process fluids with high vapor pressures. Plants typically have general rules on temperature ranges and vapor pressure margins that they want to maintain.

Both water- and air-cooled heat exchangers may be used with these plans. The use of an air-cooled heat exchanger has the advantage of eliminating water side fouling, but is less efficient and may not be suitable for low pressure differential pressure applications.

Advantages

• No dilution of process stream.
• Provides cooled process to the seal chamber, improving lubricity and increasing the vapor pressure margin.
• Can be applied to any pump where Plan 11 can be applied.

Disadvantages

• High heat load on the heat exchanger, resulting in high operating costs with the potential for fouling.
• Some fluids may congeal or become highly viscous, during idle periods, if flow is not maintained through the heat exchanger.
• Flush is recirculated.

Sizing

The flow rate should be determined in a similar manner to that done for Plan 11. Depending on the process fluid, a desired injection temperature should be determined.

For hydrocarbon based fluids, API 682 recommends 36-deg F below the vapor point. For water and other aqueous solutions, most seals require that the temperature of the flush being introduced to the seal chamber be maintained below 180-deg F.

The heat exchanger should be sized based upon the above flow rate calculation, the capability of the cooling circuit along with some safety factor for fouling.

Controlling

The flow rate should be controlled by an orifice or series of orifices in the flush line. Improved results for process fluids near their vapor pressure can be realized by locating the orifice(s) between the heat exchanger and the seal chamber. In this arrangement, the fluid is first cooled, resulting in a lower vapor pressure before the pressure drop across the orifice occurs.

Plan 23

Plan 23 is a closed loop circulation system used on hot applications for providing cooled flow to single seals. In Plan 23, a pumping ring in the seal chamber circulates product through a heat exchanger and back to the seal chamber. A throat bushing is used to isolate the cool seal chamber from the hot pump. The use of a close clearance throat bushing is recommended to minimize the mixing of the hot process fluid with the cooler seal chamber fluid.

In the past, the primary use of Plan 23 systems has been hot water services, but recently it has been become more popular in refineries, where it has been used on hydrocarbon services. In comparison to a Plan 21 or Plan 22, the heat load on the heat exchanger is considerably less. The heat load consists of heat soak from the pump, heat generated by the seal faces, along with turbulence or churning of the pumping ring and seal head within the seal chamber.

As a comparison, with a 350-deg F water application with a 3-in seal at 3600-rpm and 500-psi, the total heat load is 13,300-Btu/hr versus the 270,600-Btu/hr noted above for a Plan 21. The breakdown of the Plan 23 heat load is 6200-Btu/hr for seal generated heat, 6,840-Btu/hr for heat soak, and 260-Btu/hr for turbulence.

Advantages

• The process fluid is used to cool and lubricate the seal.
• The reduced operating temperature improves lubricity and reduces the possibility of vaporization in the seal chamber.
• Plan 23 is very efficient versus the alternative Plan 21.
• The cooler is less likely to scale or foul.
• Less direct cooling of process fluid than Plan 21.
• Can provide a cooled seal chamber thru effective thermosyphon effects when the pump is idle.

Disadvantages

• The initial cost is more than Plan 21 because of ancillary components.
• Plan 23 is not used for fluids with high freeze points or for viscous fluids because the pumping ring may not be able to force circulation.
• Venting is essential for Plan 23.
• Selection, design/location of the pumping ring, inlet and outlet ports, and piping are crucial to the successful operation of Plan 23.

Plan 31

In Plan 31 the product is introduced to an abrasive separator from the discharge of the pump. The clean fluid is routed out the top of the separator and into the seal chamber, while the process with the heavier solids is routed back to pump suction.

This plan should only be used for services containing solids that have a specific gravity at least twice that of the process fluid. Throat bushings are a requirement when using this flush plan. In this case, the throat bushing separates the clean fluid in the seal chamber from the dirty process fluid. The rule of thumb is that the velocity of the fluid passing through the throat bushing be on the order of 15-ft/sec to prevent the particles from entering the seal chamber. The flow rate of the clean flush will dictate the type of bushing required, where floating type bushings with close clearances should be used on low flow rate applications.

In an abrasive separator, the fluid containing the abrasives are fed into the inlet at the top of the cylindrical cone directed tangentially to the wall of the separator at a velocity sufficient to create a spiral or vortex action. The developed centrifugal force from the rotation of flow throws the heavy abrasive particles to the wall of the separator, where the abrasives collect and pass downward and out of the unit through the separator discharge port.

The clean fluid moves to an inner spiral and is displaced upwards and out through a vortex finder located at the top center of the unit. The proper installation of the abrasive separator is a necessity for this flush plan to work. Typically, the separator needs to have a minimum of a 15-psi differential to operate properly. The maximum particle size should be less than one-quarter the size of the inlet orifice.

It is important that the lines running from the clean outlet at top of the separator and the dirty outlet at the bottom be at similar pressures to obtain proper separation and flow rates. It may be necessary to vary the length of tubing or piping or even to add an orifice to one or both discharge lines to get the proper flow established.

The flow rate can be controlled by an orifice in the line running from the pump discharge to the inlet of the abrasive separator. This orifice should not be less than .125-in and should usually be larger, depending upon the size and the concentration of the particles being removed. Either multiple orifices or a "choke tube" can be used to control the flow.

On vertical pumps where the seal chamber is at discharge pressure a separate line needs to be added, going from the bushing at the bottom of the seal chamber back to suction to reduce seal chamber pressure to allow the clean fluid to enter the seal cavity. On small or low differential pumps no orifices are required.

Advantages

• Solids are removed from the flush stream keeping the seal chamber clean.
• Unlike a strainer or filter, the abrasive separator does not have to be cleaned.

Disadvantages

• It is sometimes difficult to obtain the desired pressure differential required for the abrasive separator to operate efficiently. Exceeding the published differential pressure will cause the separator to not function properly.
• Improper piping will cause the separator to not operate efficiently. Unless the pressure differential from the two discharges to the final sources are almost the same, the separator can either starve the seal or allow abrasives to flow into the chamber.
• The abrasive separator and the piping in the dirty outlet leg can become worn over time from the abrasives spiraling down the coned shaped bore.
• Not advisable on low vapor margin applications, as vapor bubbles have a more natural inclination to be channeled into the seal flush connection.
• Less effective with viscous fluids

Sizing

Generally the flush rate sizing will be the same as a Plan 11, keeping in mind that only the clean fluid flows to the seal. Various models or sizes of separators are available to provide different flow rates depending upon the pressure differential available.

Plan 32

In Plan 32, the flush stream is brought in from an external source. This plan is almost always used in conjunction with a close clearance throat bushing. The bushing can function as a throttling device to maintain an elevated pressure in the stuffing box or as a barrier to isolate the pumped product from the seal chamber.

Plan 32 is used when a process stream is difficult to condition in a way that will provide adequate cooling and lubrication to the mechanical seal. In addition, it is often employed when a process stream includes components, which may either result in abrasive wear or will impede free movement of critical seal components.

The design of a Plan 32 flush system involves application of hardware and logic that will provide the seal with an environment conducive to long term service, while not compromising the operation and profitability of the process stream.

Advantages

The external flush fluid, when selected and applied properly, can result in vastly extended seal life, resulting in improved MTBPM for the pump system.

Disadvantages

• Product degradation or dilution will occur when using this plan.
• Depending on overall system design, introduction of an external fluid to the process stream can result in increased energy and reprocessing costs.
• Support system costs can be very high and adds additional equipment to the system, which must be in operation whenever the involved pump is on-line.

Sizing

The flush rate is critical to any seal, but takes on another dimension when Plan 32 is involved. When an outside flush source is used, concerns regarding product dilution, and/or economics almost always surface. For these reasons, it is imperative that the seal supplier be adequately informed with regards to any limitations that will be placed on the flush rate.

With respect to the flush rate, three common scenarios should be considered. In all cases, the flow rate required to cool the seal should take precedence:

• Exclusion of process from the seal chamber is the primary objective. In this case, a close clearance, floating throat bushing should be placed in the back of the seal chamber. The flow rate which will normally achieve solids exclusion is 15-ft/sec.
• The process is at or near its boiling point and other flush plans are not practicable. Processes in this category are often hot, thus heat soak is also a consideration. When calculating the flow rate required, one should not allow more than one-quarter of the available flush fluid-temperature margin to be used.
• The process is not at its boiling point, though it has properties which adversely affect seal life. This scenario is common in process streams which have a tendency to polymerize, congeal, or set up at various stages in a batch process. In this situation, simply diluting the process is often all that is required to maintain reliable seal operation. The required flush rate may be quite low, and often actual flush rate determination may be derived more from experience. In this case, knowledge of the process and its interaction with the flush stream are key to success.

Controlling

Conditioning and controlling the rate of flush in a Plan 32 system can range from simple and inexpensive to elaborate and costly. Simpler is usually better whenever possible. Finally, the device selected to control the rate of flush is the most critical decision to be made, some methods are:

• A drilled orifice or choke tube is the simplest device and will normally be the least costly. However, careful attention must be paid to protecting the orifice from plugging, particularly if the size is less than .125-in. Also, if the supply pressure and/or box pressure are not well determined, or are variable, accurate sizing may not be attainable.
• A manually adjustable needle or globe valve with flow indicator can be employed, thus allowing "on the fly" control of the flush rate. This approach will allow accurate tuning of the flush rate, though consistent monitoring is required.
• A control valve is the ultimate control of flow rate, though cost can be substantially higher to purchase and maintain such a device.

General

Communication between the seal company and the end user is key to success of Plan 32. As with any application, the fluid properties and service conditions of the process stream must be established in order to provide a successful seal design. However, to ensure success on Plan 32 applications, fluid properties of the external flush stream should also be presented with the main process details.

When Plan 32 is applied to process streams which are hot, the flush liquid may have a higher vapor pressure than the process fluid when at the same temperature. In short, the introduction of a lower boiling point liquid into the process stream will lower the NPSHA at the impeller to some degree and will have a negative effect on pump capacity as the liquid vaporizes in the pump.

In the worst case, the pump may vapor lock or be damaged by the resulting disturbances. In this case, the flush rate is often "fine tuned" in a way that provides adequate seal cooling and minimizes vaporization within the pump. Plan 32 is not recommended for cooling only, as the energy costs can be very high. Further, material selections made for the seal should be made based on the extremes of the process fluid conditions and the external flush fluid, not the flush by itself.

Plan 41

Plan 41 is a combination of flush Plans 21 and Plan 31. The flush comes off from pump discharge, is first directed through an abrasive separator to eliminate solid particles, and then goes to a heat exchanger to reduce the temperature before being introduced into the seal chamber.

Optional accessories are an orifice to control the flow and a temperature indicator on the product outlet side of the heat exchanger. In some installations, temperature indicators are also used to monitor cooling water temperatures. This Plan should only be used for services containing solids that have a specific gravity at least twice that of the process fluid. Throat bushings are a requirement when using this flush plan.

The flow rate can be controlled by an orifice in the line running from the discharge to the inlet of the abrasive separator. This orifice should not be less than .125-in and should usually be larger, depending upon the size and the concentration of the particles being removed. Either multiple orifices or a "choke tube" can be used to control the flow.

On vertical pumps where the seal chamber is at discharge pressure a separate line needs to be added, going from the bushing at the bottom of the seal chamber back to suction to reduce seal chamber pressure to allow the clean fluid to enter the seal cavity. However, in some cases, the pump manufacturer uses the top bushing as a balance piston, so the pump manufacturer should be consulted. On small or low differential pumps no orifices are required.

It is important that the lines running from the clean outlet at top of the separator and the dirty outlet at the bottom be at similar pressures to obtain proper separation and flow rates. It will be necessary to vary the length of tubing or piping in the dirty discharge from the abrasive separator back to pump suction to obtain the same pressure drop produced by the heat exchanger.

This flush plan can be very difficult to pipe properly to obtain the correct flow rate through the abrasive separator and the heat exchanger. Due to this constraint and the potential for high heat loads as noted with Plan 21, it is not a popular option. As with Plan 21, the abrasive separator needs to have a minimum of a 15-psi differential to operate properly. The maximum particle size should be less than one-quarter the size of the inlet orifice.

Advantages

• Solids are removed and product temperature is reduced to enhance the seal's environment.
• Unlike a strainer or filter the abrasive separator does not have to be cleaned.

Disadvantages

• This plan is not suitable for very low head services as the pressure drop thru both the abrasive separator and heat exchanger may be too great.
• Piping this arrangement to get the proper pressure drops in order to get efficient operation of the abrasive separator and the correct flow through the heat exchanger is difficult. Exceeding the published differential pressure will cause the separator to not function properly.
• Depending upon the temperature of the process the heat load on the heat exchanger can be high, resulting in high operating costs for the cooling water and/or fouling of the heat exchanger.

Plan 62

Plan 62 is a common flush plan to improve the environment on the atmospheric side of single seals. Typically, this is either low pressure steam or nitrogen to prevent coke formation on hot hydrocarbon services, water to prevent the formation of crystalline substances on fluids with solids in solution, or nitrogen to prevent icing on cold or cryogenic applications.

It is typically used with a floating or segmented bushing to limit the leakage of the quench fluid to atmosphere, but can be used with a fixed bushing if the application permits. Information on quenches was introduced in the first article of this series.

Advantages

• Low cost alternative to tandem seals to improve condition on low pressure side of process seal.

Disadvantages

• Leakage of process past primary seal is not contained except with the throttle bushing. Leakage can then leak to atmosphere or go to a drain.
• Improper control of steam can allow condensation to form that can boil and cause seal damage on hot processes.
• Poor steam control can lead to a reverse pressure on the seal and/or bearing oil contamination.

Plan 65

Plan 65 is a leakage detection plan that is normally utilized with a single seal. This should be used on applications where the seal leakage would normally be in a liquid state.

From the gland drain connection, leakage is directed past or through a reservoir containing a level switch, through an orifice and finally into either a sewer or liquid collection system. If the leakage from the seal is excessive, the orifice downstream from the reservoir restricts the flow allowing the level in the reservoir to rise, setting off an alarm.

The downstream orifice should be located in a vertical leg to avoid accumulation of leakage in the pipe. This system should have a line running from the upper section of the reservoir connected to the piping downstream of the orifice to excessive leakage to drain.

The selection of a proper throttle bushing is important. A fixed bushing, especially on larger size seals incorporating larger clearances, can allow leakage to leak past the bushing. This can contaminate the surrounding area or even spray the area in the case of a severely damaged seal. If axial space is available, it is recommended that either a floating or segmented bushing be used with this plan.

Advantages

• Provides an indication of excessive seal leakage without manual inspection.
• Can provide an automatic shutdown of equipment.

Disadvantages

• Cost of system.
• Leakage levels have to be relatively high to set off the alarm.

Next month: Part Three concludes our series.

Pumps & Systems, June 2007