API 682 Seal Flush Plans
The mechanical seal is the most likely part of the pump to fail. Approximately 70% of the pumps removed from service for maintenance are victims of mechanical seal failure. Mechanical seal parts are highly engineered with very close tolerances and any upset in the pump or associated system can cause seal failure, including:
- Choked suction screen
- Rapidly closed valves
- Air or vapor entrainment
- Misalignment between pump and motor
- Pipe strain
- Service point far from the best efficiency point
- Hot fluids
- Coking or salting fluids
- Chemical composition variations
- Failing bearings
- And many more………..
Mechanical seals are based on positioning two very flat and smooth discs called seal faces, one rotating on the shaft, and one stationary in the pump, against each other. The discs are flat and smooth enough to ALMOST prevent the pumped fluid from leaking out between them. However, the faces do rely on a very thin film of fluid between the faces to lubricate that rubbing fit. Without this film of fluid, the seals will overheat and fail. Lack of lubrication is the PRIMARY cause of seal failure.
If the fluid is very hot, it can flash to vapor as the fluid moves across the faces, again resulting in a lack of lubrication. Note that gas seals use a gas film between the faces to minimize face contact and heat buildup.
Abrasives can also find their way between the seal faces and wear the face materials quickly.
Seal flush plans are intended to keep the area around the seal in the most seal friendly environment practical, usually meaning clean and cool. Dual seal plans also provide backup and leak detection for safety.
Note that seal flush plans use pressure differences at the pump to drive the flush fluids. The pump suction is low pressure, the seal chamber is a medium pressure, and the pump discharge is at high pressure.
If you have any questions about which seal plan to use on your application, please contact us.
Single Seals – Basic Heat Removal – Plans 01, 02, 03, 11, 13, and 14
As the seal faces rub together (with their thin film of lubricating fluid), they generate heat. The heat can build up in the seal chamber and push the fluid towards its boiling point, resulting in premature flashing, lack of lubrication, and failure. This first set of seal plans is intended to create circulation through the seal chamber to dissipate the heat out of the seal chamber and back into the pumped fluid.
- Uses passages built into the pump casing to direct fluid
- Flush fluid flows from high pressure at pump discharge, through medium pressure seal chamber, and back into the pumped fluid
- Just like Plan 11, but with internal passages instead of the external tubing
- Unusual for modern pumps
- Isolated seal chamber with no flush fluid moving through
- Seal chamber has heating/cooling jacket built in to add or remove heat from chamber fluid
- Used in temperature-sensitive fluids like molten sulfur
- Tapered seal chamber allows air and vapors to vent away from the seal
- Allows good fluid circulation around the seal to remove heat
- Common in ANSI style pumps
- Very effective and highly recommended for common services
- Flush fluid flows from high pressure at pump discharge to the medium pressure seal chamber and back into the main flow to remove heat from the seal chamber
- Allows the seal chamber to vent on horizontal pumps during initial pump filling
- Orifice used to limit flush fluid velocity entering the seal chamber. A high-velocity flush can erode the outer diameter of the seal faces
- Can be used to increase the seal chamber pressure. The increased chamber pressure may be required to keep chamber fluid from flashing to vapor or to provide enough pressure to push the fluid between the faces for lubrication. (Seal chamber must be 5 psi minimum above external atmospheric pressure)
- Very common seal flush plan. Can be overused in applications where other plans are better suited
- Flush fluid flows from the medium pressure seal chamber to low pressure at the pump suction
- Best plan for air, vapor, and particulate evacuation from the seal chamber
- Standard on vertical pumps
- Velocity can be limited by using orifice
- Combination of Plan 11 and Plan 13
- Fluid flows from high-pressure pump discharge, through medium pressure seal chamber, to low pressure at the pump suction.
- Usually used on vertical pumps where increased seal chamber pressure is required, or for ensuring seal faces are wet at startup
Single Seals – Flush Fluid Conditioning – Plans 12, 21, 23, 31, and 132
These seal plans are intended to provide the seal with the friendliest environment possible by cooling and/or cleaning the fluid in the seal chamber. The throat that separates the seal chamber from the main pumped fluid can be further restricted by adding a close clearance bushing in the bottom of the seal chamber, better isolating the cool, clean seal chamber fluid from the hot, abrasive fluid in the pump.
- Just like a Plan 11, but with a filter in the flush line to remove abrasives.
- Can be very simple or very complex (simplex or duplex filters, with or without instrumentation)
- Just like a Plan 11, but with a cooler in the line to remove heat. Keeps the fluid in the chamber cool
- Provides vapor pressure cushion against flashing
- Reduces coking of hydrocarbons as they cross the seal faces towards the atmosphere
- Coolers can be water-to-water (shell and tube) or water-to-air (radiator style)
- One-pass system. The hot fluid is cooled and sent through the chamber back into the pumpage. Takes a lot of cooling capacity to cool the flush fluid, and water-to-air coolers often have inadequate cooling capacity
- Similar to Plan 21, intended to cool the fluid in the seal chamber
- Rather than a Plan 21 single-pass system, a Plan 23 is a multi-pass system. Fluid comes FROM THE SEAL CHAMBER instead of the pump discharge is cooled, and directed back to the seal chamber
- Repeated cooling of the seal chamber fluid can cut cooling water demand in a water-to-water system by over 90%
- The cooling load is low enough to make a water-to-air radiator-style cooler effective
- Fluid is driven out of the chamber and through the cooler by a “pumping ring” or other “pumping feature” built into the seal. These features provide very little differential pressure. Connecting tubing must have long, sweeping bends, well vented high points, and low point blowouts to ensure fluid flows
- Like a Plan 12, but with a cyclone separator rather than a filter to separate out the solids
- Clean fluid from cyclone flows to seal chamber and the dirty fluid is returned to pump suction
- Solids need to be heavy to work effectively
- Forget all the aggressive fluids in your pump. Flood the seal chamber with an external clean, clear fluid
- Instrumentation and strainer are optional, but a flow meter with a control valve is highly recommended
- Flush fluid must be compatible with pumpage
- Makeup water lines can be used effectively for this type of flush
- Combination of Plan 21 and Plan 31. Fluid is cooled and cleaned
- For hot and dirty, abrasive services
Single Seals – Quench and Leak Detection (Outboard) – Plans 62, 65A, 65B, 66A and 66B
Quench piping does NOT change conditions inside the seal chamber, on the wet side of the seal faces. Rather, it affects or monitors the environment on the ATMOSPHERIC side of the seal faces.
Pumps that leak when they are filled, even before they are started, often have a flush line intended for a Plan 11 or 13 connected to the QUENCH port, leading to the atmospheric side of the seal. There should be a “Q” or the work “QUENCH” stamped in the gland at this port.
For flush plans Plan 65A, 65B, 66A, and 66B, facility owners may want to know if their seals are leaking excessively without going to the expense of dual seals. These seal plans direct excessive leakage on the outside of the seal to an alarm instrument. Remember that seals leak a little bit. They need to in order to lubricate the faces and function correctly. The plans below handle the nuisance leakage in different ways.
- Used in salting services like sodium hydroxide. The leakage across the seal faces will turn to salt when it reaches atmosphere. The salt crystals can wear the faces or build up in the seal, preventing the movement necessary to keep the seal faces in contact. The salt on the outboard of the seal can be washed away with a water quench through the quench and drain ports. Usually, a close clearance bushing is installed at the extreme outboard end to the seal assembly to help keep the quench fluid moving from the quench to the drain port (or vice versa) and not just run out along the shaft. Also used for slurry services
- Grease can be introduced into the quench port. This external grease can provide temporary lubrication to the seal in case the pump sees large air or vapor pockets which would normally rob the seal faces of the required lubricating fluid film
- Quench can also be gas. In hot hydrocarbon services, the fluid will turn to solid coke when it reaches the atmospheric side of the seal. The fluid would remain a liquid if the area outside the seal faces is robbed of oxygen with a flood of nitrogen or steam
- Nuisance leakage is allowed to pass through an orifice to a collection system
- Excessive leakage is backed up against the orifice and directed to a small vessel with a level switch
- Nuisance leakage AND excessive leakage are collected in a small vessel, where a high-level switch will alarm the owner
- An alarm does NOT necessarily mean a failed seal. The collection vessel might be full from years of nuisance leakage. Try emptying the vessel and observing how fast the vessel fills
- For flashing fluids (fluids that turn to vapor when released to the atmosphere)
- Two throttle bushings are used to ensure that the vapor (or fluid) leakage is limited along the shaft and out of the drain. A pressure switch picks up a rise in pressure above nuisance levels on the outboard side of the seal
- Similar to a 66A, but the leakage though the drain port is limited by an orifice in the drain port
Dual Seals – Safety backup seals for hazardous fluids – Plans 52, 53A, 53B, 53C, 54, and 55
Dual seals provide a backup seal in case the primary seal fails. They prevent hazardous fluids from leaking to the surrounding area, desirable for both environmental protection and the safety of nearby personnel.
Dual seals also capture and control any leakage of pumpage across the primary seal. The backup seal is kept lubricated by introducing a buffer/barrier fluid (often a mineral or synthetic oil, a water/glycol mix, or diesel) into the space between the primary (inboard) and secondary (outboard or backup) seals. The buffer/barrier fluid is contained in a tank (5 gallons is most common) adjacent to the pump. The instrumentation on the tank indicates what is happening with the seals.
Remember that a lubricating fluid film will flow from high pressure to low pressure. If the pump seal chamber pressure is higher than the pressure on the other side of the seal, the pumpage will be the lubricating film. If the pump’s seal chamber pressure is lower than the external pressure, the external atmosphere will migrate into the pump.
Pumps under vacuum cannot use an ordinary single seal, since air from the atmosphere would be drawn between the faces, causing them to run dry and fail. Using a dual seal allows fluid to be present at the outside of the seal. In a pump under vacuum, the buffer fluid would be pulled into the pump between the seal faces, keeping the inboard seal well lubricated.
The basic differentiation between dual seals is which way the fluid is flowing across the inboard faces.
- If the pump seal chamber pressure is higher than the BUFFER fluid between the primary and backup seal faces, then the pumped fluid will flow from the high seal chamber pressure into the low-pressure buffer fluid. This is called a DUAL UNPRESSURIZED seal (formerly called a tandem seal), and the fluid is called a BUFFER fluid.
- If the pump seal chamber pressure is lower than the BARRIER fluid between the primary and backup seal faces, then the barrier fluid will flow across the primary seal from the space between the primary and backup seals into the pump. This is called a DUAL PRESSURIZED seal (formerly called a double seal), and the fluid is called a BARRIER fluid.
Dual unpressurized system
- Buffer fluid circulates from the buffer fluid reservoir, through the space between the primary and backup seal, and back to the reservoir. The fluid is circulated by a weak pumping action built into the seal
- Any pumped fluid is captured in the buffer fluid and carried to the reservoir
- If the fluid flashes to vapor at low pressure, the vapor is piped to a flare or vapor recovery system, through an orifice at the top of the tank. If the primary seal is allowing too much leakage, the vapor will build pressure in the reservoir against the orifice and a pressure instrument can alert the operator
- If the fluid remains as a liquid under low pressure, any leakage will cause the fluid level in the buffer tank to rise, where a high-level alarm can be tripped. Just because the high-level alarm is tripped does not mean that the primary seal is failing; it is the rate of leakage filling the tank which matters. The high level may have been reached after collecting years of nuisance leakage. Often, an oil change to the original level is all that is required. Be sure the fluid is disposed of properly
- Seal face friction or hot pumpage can add heat to the buffer fluid. A cooling water coil is often installed in the reservoir to cool the buffer fluid
- A low-level switch on the seal reservoir can indicate that the backup seal is failing, allowing the buffer fluid to leak out
Dual pressurized system (seal barrier fluid is at a higher pressure than the pump seal chamber). Pressurized systems are used to ensure that very dangerous fluids remain in the pump. The difference between 53A, 53B, and 53C is the method of pressurizing the barrier fluid. Pressure in the barrier fluid should be at least 10 psi over the pressure in the pump seal chamber.
- Barrier fluid circulates from the barrier fluid reservoir, through the space between the primary and backup seal, and back to the reservoir. The fluid is circulated by a weak pumping action built into the seal
- Barrier fluid crosses the primary seal faces into the pumpage and must be compatible with the pumped fluid
- A low-level alarm in the reservoir alerts the operator that a seal may be failing, allowing the barrier fluid to enter the pump through the primary seal or the atmosphere through the backup seal
- Pressure is provided to the barrier seal reservoir by the plant nitrogen or air system
- Usually limited to 100 psi barrier pressure or 90 psi seal chamber pressure
- Pressure is provided by a bladder accumulator tank
- Fluid cooling needs to be added to the piping loop, either with a water-to-water shell and tube type cooler or an air fin radiator
- Pressure is provided by piston accumulator/amplifier
- Pressure differential across the seal is 10 or 20% higher than the seal chamber pressure (racks with seal chamber pressure)
- Allow 15-20 psi to overcome the friction required to move the piston
- Pressurized dual seal system where pressurizing, cooling, and filtration of the barrier fluid is provided by an independent oil circulation system
- One pressurizing system can be used to serve many seals
- Unpressurized dual seal system where circulation, cooling, and filtration of the buffer fluid is provided by an independent oil circulation system
- One circulating system can be used to serve many seals
Dual Gas Seals – Plans 72, 74, 75, and 76
Seal faces can be designed to maintain a gas film between them rather than a fluid film. These piping plans are intended to work with theses gas film (dry running) seals. Plan 72 and 74 bring the buffer or barrier gas into the seal; plans 75 and 76 are for the gas exiting the seal.
Unpressurized gas buffer
- The secondary seal is ordinarily running with a gas film between the faces. When the primary seal fails, the pumped fluid will fill the space between the primary and backup seal. The backup seal is now working as a liquid seal rather than a gas seal and is designed to run for about 8 hours, allowing the operators time for an orderly pump shutdown.
- Plan 72 must be used with Plan 75 or 76 to allow the gas to flow through the seal
- Plan 72 buffer gas flow keeps the gas in the seal from becoming concentrated from nuisance leakage over time so that any leakage from the gas backup seal is mostly inert flush gas and not toxic pump vapors
Pressurized gas barrier
- Both primary and backup seal are dry-running gas seal designs
- Inert flush gas will pass through the primary seal into the pumped fluid. Must be compatible
- No Plan 75 or 76 required to carry gas away
- Directs unpressurized gas exiting a seal into a collection chamber for fluid capture
- The nuisance level of flashing fluids is routed through an orifice to a flare or vapor recovery system. Excessive leakage of flashed vapors create backpressure against the orifice which can be picked up with a pressure switch or transmitter
- Non-flashing fluids are collected in a tank with a level switch alarm
- Like Plan 75, but without fluids collection tank. For use where fluids flash completely