Seal partnering 13-10
The latest fad to hit the process industry is “partnering,” allowing industrial academics to have a wonderful time thinking up these new projects so they will appear to be doing something productive and useful. Although “partnering” implies many lofty benefits it always ends up being nothing more than the purchasing department negotiating a large discount from the seal manufacturer. The really important stuff that is costing you the most money, always takes second place.
Modern mechanical seal designs are supposed to run until the only sacrificial part (usually the carbon seal face) has worn away. In better than 90% of the cases this never happens. Through some convoluted thought process, or maybe a hard sell by their local seal supplier these same academics have decided that purchasing these failing seals at a discount will somehow make sense to whomever they have to answer. I know they’re not going to like hearing this, but mechanical seals are really not a commodity. Anything with a 90% plus failure rate would have a hard time being put into a commodity status by anyone’s definition.
If you serious about trying to finally getting some decent mechanical seal life in your rotating equipment, you’re going to have to start looking for some very specific features in your mechanical seals and some very knowledgeable people to deal with when it come to application and troubleshooting. Consulting, application, and troubleshooting expenses have traditionally been built into the price of the mechanical seal. Large consumer discounts eliminate that service.
There are three things you would like to receive from your seal supplier:
- An excellent product
- A low price
- Good service, including: partial inventorying, application engineering, and trouble shooting assistance.
Choose any of the above two. Isn’t it kind of stupid to think you’ll get all three!
We will start with the product. Decide on the type of seal you’ll need :
- Stationary mechanical seals are better than their rotating cousins. Some of these stationary designs are available with seal wear indicators and you should take advantage of that feature. Stationary seal design do not have rotating springs or bellows.
- Dual seals are the only reliable method you have of protecting yourself against an unexpected seal failure, and all the problems associated with product leaking out of the pump. With all the current talk about fugitive emissions, dual seals are becoming a necessity in many applications. They are available in both the stationary and rotary version with liquid or inert gas buffer, or barrier fluid circulating between the seals, and can be installed in four different configurations:
- Back to back rotating seals are never acceptable. Back to back stationary seals are OK because the sealing fluid is at the inside seal outside diameter
- Tandem or series is the best choice for rotating or stationary seals
- Face to face designs are acceptable
- Concentric, or one inside the other is alright but this design takes a lot of radial room. Too much for a conventional pump..
- If you’re going to use single seals, the split version should be your first choice. There is no sense in taking equipment apart and spending all the money associate with disassembly, spare parts, new gaskets, realignment, insulation, etc, if you do not have to. In most cases the only problem with the pump is it is leaking. Fix the leak, do not overhaul the pump. Remember your grandfather’s words “if it aint broke don’t fix it”.
- Cartridge seals not only make assembly easier and faster, but they are your only alternative if you intend to adjust the impellers used on the common open impeller, back pullout pumps. It is unbelievable the number of companies that are failing to adjust these impellers and as a result are running the pumps very inefficiently. This can only be explained by the lack of knowledge that industry has about how pumps work. Otherwise, it is being done on purpose. Split seals can be cartridge mounted without too much trouble.
- If your seals are going to incorporate elastomers make sure you use O-rings. Stay away from wedges, chevrons, U-cups, etc. They destroy (frett) shafts and are seldom needed in modern seals.
- Metal bellows seals are good for eliminating the need for elastomers. They make sense in cryogenic service and hot applications such as polymers and resins. You can use them in any fluid other than hot petroleum products that “coke” and fail seals prematurely. Oils must be cooled.
Seal materials are an important consideration. With few exceptions you should be able to install the same seal in every pump of the same shaft size and reduce your inventory considerably. Make the decision right now to quit using “mystery materials” identified only by a part number or some generic term used to describe the material in your expensive mechanical seals. How are you going to fix something if you not know what it is? There is a lot to know about materials and it is a subject that is always changing as new materials are developed.
- If you are going to use a carbon/ graphite face use the three impregnate, non filled type in most of your applications. There will be a couple of exceptions for hot dry air and cryogenic service, but exceptions are always easy to handle.
- Hastelloy “C” springs and metal bellows plates are necessary to prevent chloride stress corrosion problems associated with the three hundred series of stainless steel. Never use stainless steel springs or stainless steel metal bellows in any mechanical seal application.
- Remember that reaction bonded silicone carbide will be attacked by high pH materials such as caustic. You will probably be better off with the alpha sintered version of silicone carbide in these applications.
- Your seal materials should not be subject to “shelf life” problems. This is an on going problem with Buna N O-rings and rubber bellows.
Look for non-clogging features in you seal designs because there are solids present in most of your applications:
- The springs should be out of the fluid unless you have some real good reason for putting them there.
- Dynamic elastomers (moveable O-rings) should always move to a clean surface.
- Centrifugal force should throw solids away from the seal moveable parts and lapped faces.
- Inconel bellows make a lot of sense in high temperature seal applications.
- A non-stick coating on sliding components helps. Baked on Teflon® works well on frying pans and seal components. The coating will also reduce hysteresis problems with sliding or flexing dynamic elastomer or O-ring.
There are other desirable features you should look for:
- Hydraulically balanced seal faces for vacuum and varying stuffing box pressures.
- The inner seal of a dual seal should be hydraulically balanced in both directions because pressures commonly reverse in these applications.
- The seal design you specify should be able to seal if it rotates either clockwise or counter clockwise. Most hydrodynamic gas seals and some single spring liquid seals cannot do this.
- The seal should have good axial and radial movement capability. This is especially important for mixer applications. Specify excessive motion designs for mixers, agitators, sleeve bearing equipment, etc. These designs incorporate:
- Wider hard faces so that the thinner carbon/ graphite face has more radial movement capability.
- More internal clearance in the seal components.
- Axial movement capability without compressing or extending the spring or springs and affecting proper face loading. You need at least 0.125 inches (3mm) in both directions. More would be better.
- The seal design you select should be the thinnest and shortest seal that satisfies the operating conditions. There is no advantage in having the seal take up too much axial or radial room. A high percentage of requests for special seals means shorter or thinner designs.
- The seal should be installed close to the pump bearing to eliminate the affect of shaft deflection problems.
The seal should be easy to install.
- No print should be required. Cartridge and split seal both have that advantage.
- The design should be independent of shaft finish and reasonable shaft diameter tolerances. In other words there should be no dynamic elastomers or metal bellows vibration dampers rubbing on the pump shaft or sleeve.
- The seal should not frett or damage the shaft or sleeve through normal operation. This will eliminate all spring loaded elastomers and many rotating seal designs.
- The seal should be packaged to protect the lapped faces. Bubble packaging on a piece of cardboard should not be allowed. The seal should be in its own box and protected by foam or a similar protective material. The lapped faces should be dipped into a protective coating that can be removed just prior to installation. A boxed seal should be able to survive a one meter (39 inch) drop without injury to the seal components.
The seal should generate very little heat.
- The seal should be hydraulically balanced for low face loading.
- Specify two way balance for the inner seal of a dual seal design.
- Low friction faces are a necessity. A good grade of carbon/ graphite running against a hard face like silicone carbide will generate very little heat.
- Use materials that conduct heat well. Silicone carbide is great.
- Never insulate the seal faces by elastomers or plastics if you can avoid it. You want to conduct the face heat into the gland or face holder when ever possible.
- Try not to position the dynamic elastomer in the seal face. The dynamic elastomer is the one part of the seal that is the most sensitive to heat.
- If you are sealing a non lubricating fluid (liquid or gas) use a dual seal with a lubricant circulating between them. Avoid oil as a buffer or barrier fluid, the specific heat of oils is too high and they all have poor conductivity compared to other cooling fluids.
- Install a gland vent above the seal faces in all vertical pump applications. Air will collect in the top of the stuffing box causing the seal faces to run dry. You can drill a hole in the upper corner, at the end of the stuffing box to vent the seal cavity in a horizontal pump.
- Use a large inside diameter, bored out stuffing box to give the seal plenty of clearance on its outside diameter. Avoid tapered stuffing boxes.
- Use heavy duty seal designs for greater pressures. These designs incorporate:
- Fiber or Teflon® back up rings to prevent elastomer extrusion.
- Thicker cross section components to prevent face distortion.
- New thinner cross section configurations have become practical since the introduction of finite element analysis programs.
- Lower spring pressure at the seal faces.
- A different balance ratio. Especially with fluids having a specific gravity below 0.4.
- Do not glue carbon/ graphite seal faces into a metal holder. The glue may not be compatible with the sealing fluid or the solvent/ steam used to clean the lines.
- Avoid “shrunk in” carbon faces. “Pressed in” faces shear to conform to the “out of roundness” of a metal holder.
Some cartridge seal feature to look for.
- The cartridge sleeve must be sealed at the inboard end to prevent solids from lodging between the cartridge sleeve and the pump shaft making seal removal difficult if not impossible.
- In some dual seal designs, if the cartridge gland is pushed to move it against the stuffing box face, the friction between the seal sleeve elastomer and the shaft outside diameter will cause the inner seal to compress and the outer seal will unload. Look out for this design flaw.
- Can the clips or whatever is setting the correct installation length be reinstalled when an impeller adjustment has to be made? In many designs it is very difficult to re-install these clips.
- Can the assembly be rebuilt easily and at a sensible cost? Does it have to be returned to the distributor or manufacturer? New “right to know” laws are going to discourage returning hardware that has been exposed to chemicals.
- The cartridge gland should have a flush port and vent built into the gland. An API type gland would be ideal with its disaster bushing and quench/ drain connection.
All split seals are not alike:
- Will the design seal both pressure and vacuum. Many designs cannot do this.
- Be sure there are no glued elastomers, especially the dynamic elastomer or O-ring that has to flex and roll to compensate for face wear and shaft movement. The glue creates a hard spot that will leak.
- Stationary designs are the best. In this design the spring or springs do not rotate with the shaft.
- Can it be mounted on a cartridge so that you can make pump impeller adjustments or be able to remove the split seal with out having to drain a side entering mixer?
- Does it have a wide hard face and lots of internal clearances for radial movement? Many of these seals are used on mixers and this is a necessary feature in a mixer application.
Once the proper mechanical seal has been specified for the application, the really difficult part takes over. Someone has to do the application for the individual chemicals and troubleshooting of failed seals. Because these people are rare, we see consumers throwing seals at the application hoping one of them will work. The 90% failure rate you presently experience is one of the results of this approach.
It would be wonderful if you could call in three or four seal companies selling the same product, with the same level of expertise and choose among them for your best price. The fact is you’ll be lucky if you can find one good supplier, and I will bet he will not be the lowest price. He will likely have the highest inital cost. Isn’t it kind of naive to think otherwise?
I know you are not going to pay attention to this, so good luck with your 90% + failure rate.