The sealing of liquid slurries 3-4
A slurry is defined as: Solids suspended in liquid that cannot be dissolved by controlling the temperature and/ or pressure. The solids may or may not be abrasive. It does no good to try to identify the number of solids or their size because no one knows how these numbers relate to slurry related seal problems. Whenever you deal with slurries there are several problems you must consider:
- The slurry can clog the flexing parts of a mechanical seal, causing the lapped faces to open as a result of both shaft and seal movement.
- If the slurry is abrasive it can wear the rotating components. This can be a serious problem with thin plate metal bellows seals.
- The pump rotating assembly will go out of balance as the slurry wears the impeller and other rotating components. This will cause excessive moving of the seal components.
- The pump will lose its efficiency as critical tolerances wear rapidly. This can cause vibration and internal recirculation problems. The wear will also cause the need for frequent impeller adjustments that will cause problems with mechanical seals
It’s generally believed that the main problem with slurries is that they penetrate between the lapped seal faces and cause damage. Although this is true, it’s also true that they cannot penetrate until the seal faces open.
Seal faces should be lapped flat to within three helium light bands. That is a distance just a little bit shy of one micron. Compare this to the fact that the smallest object that can be seen with the human eye is forty microns in size, and you’ll appreciate the technology used in the manufacture of mechanical seals. As a matter of comparison, look at a common coffee filter. It filters out particles larger than ten to fifteen microns.
All of this means that the seal is in fact a superior filter and as long as you can keep the two lapped faces in contact, there’s little chance for solids to penetrate the faces and do any type of damage.
There are three approaches to the sealing of solids :
- Design a seal with non clogging features.
- Create a clean sealing environment for the mechanical seal.
- Do a combination of both
Let’s look at each of the approaches, and in the process learn a sensible method of sealing a slurry.
Build a seal with non clogging features.
- Take the springs out of the sealing fluid.They cannot clog if they’re not in the slurry.
- Make sure the sliding or flexing components move towards a clean surface as the seal faces wear.
- Take advantage of centrifugal force to throw the solids away from the sliding/flexing components and lapped seal faces.
- Use a non stick coating to prevent the slurry from sticking to the sliding components.
- Use only balanced seal designs. Additional heat generated at the seal faces can cause many products to solidify, coke, and crystallize creating even more solids problem.
- Metal bellows designs can be used, but they must have extra thick plates to resist excessive wear. Extra convolutions will have to be provided to compensate for the higher spring rate caused by these thicker plates. Rotating the abrasive fluid with the bellows can be a big asset. Some commercial designs have this feature.
Create a clean sealing environment.
Give the seal as much radial room as possible. You can either bore out the packing chamber or install a large bore sealing chamber. Try to give yourself at least 1 inch (25 mm. ) radial space if possible. The more room you can provide for the seal the better off you’re going to be.
Try to remove the solids from the sealing area. There are a number of techniques for doing this. Some work and some do not. Let’s look at each of them. First we will look at the solutions that do not work very well and comment on their problems :
- Bad Solution #1. Connect a filter in the line from the pump discharge to the stuffing box. Since the discharge is a higher pressure the flow of liquid through the filter will clean up the fluid and then there will be clean liquid flowing to the stuffing box..
- Comment : The problem with this idea is that the filter will clog and no one will clean it.
- Bad Solution #2. Install a cyclone separator into the line instead of a filter. Connect it between the pump discharge and suction with the third (the center) port connected to the stuffing box.
- Comment: This idea is just as bad. The cyclone was never intended to be a single pass device. They work well if used in a bank of several filters but there is not enough pressure differential between the suction side of a pump and the stuffing box for them to be effective.
- Bad Solution #3. Install the seal outside the stuffing box so the springs will not be located in the dirty fluid.
- Comment: The problem with this idea is that as the seal faces wear they must move forward and in doing so they will move into the dirty fluid. The result will be that the movable face will hang up in the solids and the faces will open. Another problem with this approach is that centrifugal force throws the solids into the seal faces and not away from them.
- Bad Solution #4. Install a double rotating seal in the “Back to Back” configuration with a higher pressure, clean liquid barrier between the seals.
- Comment: This is a very common approach to the problem and has all of the problems associated with installing the seal outside the stuffing box. In addition to a rapid failure you will also experience product dilution as the barrier fluid leaks into the pump.
- Bad Solution #5. Since we are discussing things that don’t work we might as well try two hard faces.
- Comment: Needless to say they will not prevent the faces from opening and when they do open, experience shows that you’re going to destroy both hard faces. Some seal salesmen may even try to convince you that the seal faces are designed to “grind up” the solid particles into a fine powder. In other words the seal is designed as some type of a “quasi- milling machine”
Now we will look at some methods that do work:
Good Solution #1.
- Flushing with a clean liquid is a good method of cleaning up the pumping fluid. The amount of flushing you will need depends upon the design of your seal. If the design has multiple small springs in the fluid then more flushing will be required. There are various sources for the flushing liquid :
- Finished, clean product or one of the mixture’s clean ingredients
- A compatible fluid.
- A solvent.
- An additive that is going to be added down stream and could be injected into the stuffing box location.
- Clean water.
- A compatible grease can be used with most balanced seals running at lower speeds
- NOTE : Never introduce live steam into the stuffing box as it could cause the product to flash and the pump to cavitate.
- Be sure to start with a flushing pressure that is at least one atmosphere (15 psi or 1 bar) higher than the stuffing box pressure. You can use a pressure gauge to determine stuffing box pressure. You can then use a flow meter to regulate the amount of flushing fluid.
- With intermittent service pumps it is a good idea to have an electrician install a solenoid valve with a delay switch that would allow the flushing fluid to come on thirty seconds prior to the pump starting and to leave the flushing valve open for a few minutes after the pump has stopped
Good Solution #2.
- Install an oversize, jacketed sealing chamber and “dead end” the fluid. “Dead ending” means that there are no circulation lines coming in or going out of the sealing chamber.
- You can use the cooling jacket to remove the heat being generated by the seal faces as centrifugal force cleans up the solids that are present in the small amount of fluid, trapped in the seal chamber. This solution works exceptionally well with fluids where temperature control is important. Heat Transfer Oil is a prime example.
- If the fluid you’re sealing is not hot, the cooling jacket will not be necessary. Some times one shot of clean liquid into this oversize, dead ended stuffing box, is all that is necessary to seal even a severe slurry. Needless to say this application works best on a continuous running pump.
- If the specific gravity of the solids is less than the liquid they are mixed in, centrifugal force will not work for you. A clean flush will be necessary in this instance or one shot of a higher specific gravity compatible liquid.
Good Solution #3.
- If the solid particles are sub micron in size two seals with a higher pressure barrier fluid becomes necessary. In some instances you might want to use two hard faces on the inner seal. Kaoline and some dyes are a good example of products with sub micron size particles.
Good Solution #4.
- Install a large seal chamber on the pump and connect a recirculation line from the bottom of the stuffing box back to the suction side of the pump. The size of this line will be determined by the size and number of solids that you’re trying to remove.
- This will cause liquid to flow from behind the impeller to the stuffing box and then on to the suction of the pump. Fluid entering the stuffing box, from behind the impeller has been centrifuged and should be a lot cleaner than the fluid you’re pumping. This solution works well with closed impeller pumps and those open impeller designs that adjust to the front of the pump volute. If your open impeller adjusts to the back plate (as is the case with the Duriron pump) this method is not as effective.
- Do not use this technique if:
- You’re pumping close to the vapor point of the fluid, as lowering the pressure could cause the pumping fluid to vaporize in the stuffing box and in some cases between the seal faces.
- You’re sealing a Duriron pump where the impeller adjusts to the back plate.
- You’re sealing double ended pups where the stuffing boxes are at suction pressure.
- If the solids have a low specific gravity or density and float on the liquid.
Compensate for the fact that the rotating unit will go out of balance.
- The seal faces have to be vibration dampened. O-ring type seals are equipped with a natural vibration damper because of the dynamic elastomer that has been installed. Metal bellows seals have to be provided with some other method. Letting the seal face holder rub and vibrate along the shaft is an approach used by most metal bellows seal manufacturers. The logic is questionable.
- Give the seal room to move. Shaft run-out and vibration can cause the seal rotating components to contact the inside of the stuffing box unless you have installed an oversized sealing chamber.
- Use motion seals if the run out or vibration is excessive. Unlike pump seals, these seals have much wider hard faces and extra internal clearances. Most popular designs can compensate for plus or minus 1/8″ (3 mm. ) in a radial direction and 1/8″ (3 mm) in an axial direction.
- Move the seal closer to the bearings. Split seal designs are a logical choice because most of them come with a stuffing box extension gland that positions them next to the bearings. A support bushing, or sleeve can be installed near the face of the stuffing box to minimize the affects of unbalance, vibration and shaft whip or wobble. A variety of materials are available for these support sleeves. Check with your supplier for availability in your area.
The pump will lose its efficiency and experience more shaft movement as close tolerances wear.
- If you’re using open impellers it will mean frequent impeller adjustment. In this case a cartridge seal is your best approach as impeller adjustments can be made without disturbing the seal face loading. Split seals can compensate for the initial impeller setting and split seals mounted on a split sleeve will easily compensate for movement caused by temperature growth or impeller adjustment.
- Closed impeller pumps will have to be disassembled and the wear rings changed when the clearances become excessive. If you’re fortunate enough to have adjustable wear rings on your pump then only an outside adjustment will be needed and the pump will not have to be put out of service. Cartridge seals can almost always be reused in these applications because the seal faces were not separated as the pump was disassembled.
- Remember that with closed impeller pumps the wear rings will have to be replaced when the normal clearance doubles. A typical normal clearance would be 0.008″ to 0.015″ ( 0,2 to 0,4 mm). A good rule of thumb is that the pump will lose 1% of its capacity for each .001 inch (0,025 mm.) of wear ring wear.
A few more thoughts about the sealing of slurries :
- Kaoline (China clay) is a product that’s used in many industries including paper and pharmaceutical. Its abrasive particles are less than one micron in size and as a result, will penetrate lapped seal faces causing rapid carbon and hard face wear. In this application it is necessary to use two mechanical seals with a higher clean barrier pressure between the faces to prevent most of the penetration.
- In addition to one of the recommended solutions mentioned above, two hard seal faces can also be used as some particles will always penetrate the faces.
- Using a combination of packing and a split mechanical seal is proving to be an ideal solution in many applications. With the seal installed there is no pressure differential across the packing and therefore the solids do not try to penetrate. Move the packing flushing line to the bottom of the split seal housing and flush the packing through this connection instead of the lantern ring or seal cage. The flushing is necessary to remove the additional heat being generated by the packing.
- You should be able to cut the flushing fluid volume down to about one third of the amount you had been using. Since the packing is not being forced to the shaft only a small amount of cooling is necessary.
- CAUTION! It is important that the flushing fluid be kept at a higher pressure than the stuffing box pressure. If this pressure differential fails it could force the packing into the rear of the mechanical seal. A split adapter plate installed between the split seal and the stuffing box face can prevent the packing from blowing out if the flushing pressure is lost.
- If you elect to use a rotating metal bellows in a slurry application, remember that the bellows should rotate the fluid in the sealing chamber. Most bellows designs allow the thin bellows plates to cut through the abrasive slurry and experience severe wear and breakage in a short period of time.
- On February 13, 2018