Centrifugal Pump Seal Basis: Mechanical Seal Washing

Outlet recycling

 

A line connecting the pump outlet to the stuffing box. The high-pressure fluid (flushing fluid) at the pump outlet enters the stuffing box near the mechanical seal end face to flush the end face, and then circulates through the stuffing box to the pump body. This technique poses several problems for maintenance personnel:

1) If the fluid contains solids (which most fluids do), the centrifugal action of the impeller will concentrate the solids on the inner diameter of the pump volute, and it is this dirty fluid that is recirculated into the stuffing box. Needless to say, this will not be conducive to the reliable use of mechanical seals, because solid particles will act like "sandblasters", cut into the lap sealing surface and clog the sliding seal components.

2) When solids pass through narrow gaps, pump wear rings, critical tolerances, and close-fitting bushings will experience rapid wear.

The only legitimate use for this flushing is to pressurize the stuffing box to prevent the liquid from vaporizing. Be careful if using this method in hot water applications, especially if a heat exchanger is installed in the recirculation line. Leakage of hot water or steam in any connection can be dangerous to anyone in the area, while solids can also clog the heat exchanger.

When using this line to pressurize the stuffing box, the following points should be kept in mind:

1) Install a tight-fitting bushing (i.e. throat bushing) at the bottom of the stuffing box with a clearance of 0.002 inches/inch (0.002 mm/mm) of the shaft diameter.

2) If metal bellows are used for sealing, ensure that the line is removed from the lap sealing surface and the thin metal sheet.

3) If a properly installed, balanced O-ring is used (which should be done), the sealing product will not flash between sealing surfaces as long as the stuffing box pressure is at least one atmosphere higher than the liquid vaporization pressure. The outlet recirculation line should be able to guarantee this pressure difference.

 

Suction recirculation

 

A line connected between the suction of a pump and the bottom of the stuffing box or seal gland. Many pumps already have a fitting in the suction inlet for mounting the suction gauge. However, if no suction recirculation line interface is available and the pipe wall is not thick enough to drill and tap, an interface can be installed on the pipe or pipe flange.

The pressure of the stuffing box is almost always higher than the suction pressure of the pump. The liquid behind the impeller will circulate through the stuffing box to the pump inlet. This liquid has been centrifuged by the impeller, and the result is that the liquid in the stuffing box is much cleaner than the one pumped. In many cases, the introduction of clean liquids and dilution of process liquids can be avoided.

This environmental control is very effective in closed impeller pump designs and those with open impeller designs that are adjusted towards the pump volute rather than the back plate, such as Duriron pumps.

 

 

Washing

 

At one atmosphere (15 psi./1 bar) above the stuffing box pressure, clean liquid from the outside is introduced into the stuffing box through a regulating valve. The liquid should be introduced from the bottom of the stuffing box to ensure thorough cleaning. All of these liquids end up in your product.

If you use a balanced O-ring, only enough liquid is needed to remove solids that may interfere with the seal movement. No additional liquid is required to provide cooling because the balance seal does not generate enough heat to cause problems in most applications.

For spring-loaded mechanical seals immersed in liquid, only 1 to 2 gallons (4 to 8 liters/hour) of flushing is required per hour. Note: This is a rinse per hour, not per minute. If using a multi-spring design, consult the manufacturer for advice. Cleaning fluids can come from the following sources:

 

1) Clean water

2) Compatible liquid

3) Solvent

4) Pump an ingredient in the product

5) Clean finished products that will not harm the raw materials

6) An additive that will be placed downstream of the product can be added at the pump stuffing box

If you use shop water to flush, you must be careful, otherwise solids in the flush water will clog the flow control valve. Shop water pressure also varies throughout the day and in some cases may be lower than the pressure in the pump stuffing box.

 

 

Spacer or buffer

If two seals are used in one application, liquid needs to be injected between them. If the liquid between the seals is higher than the stuffing pressure, we call it the isolating liquid. If it is below the stuffing box pressure, we call it a buffer liquid. Liquids can be circulated by forced circulation, pumping rings, or convection. The specific method used will be determined by the pressure, pump speed and shaft size. All seal manufacturers have available charts to provide you with the correct selection guide.

If you choose to use a forced-circulation system, ensure that fluid is brought in from the bottom connection of the stuffing box and discharged from the top connection. This arrangement will ensure that the space between the seals is vented and properly cooled.

Forced circulation is the recommended method for all vertical shaft applications, although when the liquid circulating in the seal changes speed at the convective tank connection, it may counteract the central position of the seal gland and gain a small pumping effect. Please consult your local distributor for an explanation of this principle.

Many of the latest seal designs use built-in pumping rings to enhance convection. This pumping arrangement is necessary when oil is used as the isolation liquid. The figure below shows a typical convection system that can be used with two balanced seals.

Water is one of the best isolating or buffering liquids because it has a high specific heat and good electrical conductivity. Oil is probably one of the worst because of its low specific heat and poor electrical conductivity. Keep this in mind when selecting isolation or buffer for a seal.

The type of seal chosen will determine whether the isolation liquid must be kept above or below the stuffing box pressure. In this case, pressure fluctuations are normal, so a bidirectional balanced seal should be selected to eliminate any problems that may arise when isolating the liquid or when the system pressure changes.

Make sure the convection tank or forced lubrication system is connected so that the inlet is at the bottom of the double seal and the outlet is discharged from the top of the seal. This arrangement will allow the exhaust to be sealed and ensure that the passage is filled with liquid.

 

Jacket (B)

 

High temperature pumps have a cooling/heating jacket installed around the pump stuffing box. If a jacket is not installed on the pump, it can be purchased from the pump manufacturer or an "after market" supplier.

The secret to using a jacketed stuffing box is to install a thermal sleeve at the bottom of the stuffing box and then "dead end" the stuffing fluid. Dead ends mean that suction or discharge recirculation lines should not be installed. Any material with poor thermal conductivity can meet the requirements of the casing, provided it is compatible with the pump product being sealed. Carbon is a good choice because, unlike teflon, it doesn't change size much as the temperature changes.

A small amount of liquid or steam through the jacket can control the stuffing box within the desired temperature range. In some cases, cold heat transfer oil is used. Keep in mind that the jacket also provides cooling for the bearing box and stuffing box.

Make sure the jacket fluid does not contain calcium (hard water) or anything that could form a film on the inside of the jacket surface and limit heat transfer. If you experience this problem, you can use some cleaners. Condensate is a good jacketed circulating liquid with few problems.

 

Quench

 

Quench: Q interface on API gland.

1) Some seal caps have an exhaust port or quench connection behind the seal to control the temperature of the sealed area with steam or other fluids. Tight-fitting carbon (or any other non-spark material) bushings (throttling bushings) are installed on the outside of this connection to provide tight clearance between the gland and shaft.

2) Refinery applications use a quench cap and call it an API (American Petroleum Institute) cap.

Now that you know the names of the six different ways, let's look at how to use them in various sealing applications:

 

Outlet recirculation

 

1) It can be used to pressurize the stuffing box to prevent vaporization of the product.

2) A heat exchanger can be installed in the line, but only when the pump is running.

3) Do not install a filter in this line, as it will clog and limit the recirculation of the stuffing box.

4) Many consumers have installed a "cyclone" type device in the recirculation line. These separators have never been shown to be very effective at removing solids from stuffing fluid.

 

Suction recirculation

 

1) The vertical pump exhausts through this line.

2) Use the line for normal product recirculation and replace the stuffing box fluid with a cleaner fluid recirculated from behind the impeller.

3) This connection provides a safe way to empty the stuffing box before removing the seal.

 

 

Washing

 

1) Introduce the clean liquid into the stuffing box to remove solids or any problem fluids.

2) Rinse with the cooler liquid to cool the hotter liquid.

3) Remove liquids that are sensitive to temperature and/or pressure changes.

4) In dual-seal applications, this connection can be used to cross-connect the stuffing box, thereby equalizing the pressure in the stuffing box.

 

Spacer or buffer

 

1) Prevent pressure drop of inner seal.

2) Protect dynamic O-rings in ethylene oxide applications.

3) Control the temperature of the sealing surface.

4) Grading pressure in high pressure applications.

5) Keep air or oxygen away from the sealing surface.

6) Detect internal seal leaks when used with convection tanks.

7) When using the inside seal to seal non-lubricant, transfer the load to the outside seal.

8) This is the conventional method of sealing gas.

 

jacket

 

1) The best way to control the temperature inside the stuffing box when the pump is shut down.

2) Ensure that no suction or outlet recirculation lines are connected.

 

Quench or exhaust and drain - add disaster bushing

 

1) If the bearing fails, the disaster bushing will protect the seal from hitting the inside of the stuffing box. This is a very important feature in applications where the product can burn or explode when overheated.

2) In the event of a major seal failure, the disaster bushing will protect site personnel from injury (preventing a large leak of the pumping medium). Most leaks can be connected to a collection tank or field drain line via a drain interface.

3) Wash away the solid material on the outside of the seal to prevent seal wear and "hanging" when the seal moves forward.

4) Wash away toxic or corrosive vapors that may leak from the sealing surface.

5) Control the temperature of the sealed area.

6) As a backup in case the heating/cooling jacket fails