Seal and Pump Specifications
Communicating with suppliers 8-3
The precise meaning of words continues to be the major obstacle when technically trained people try to communicate with non-technical people, and the sales representatives of outside vendors.
Sometimes the results are funny:
I have worked with overseas translators for more than twenty five years and have had “hydraulic ram” translated as “wet sheep” and one time when I told my students that “failure of this course would require attendance at a make up school”, it was translated as a school where you would be taught how to put on face powder, lipstick, eye shadow. etc.
The problem is not limited to international dialogue, it occurs frequently between people using a so called common language. The result can be frayed nerves, extra cost, unexpected down time and costly failure. Here are a couple of examples of what I am talking about. Do you do this?
The word “documentation” has multiple meanings and is often confused with “material identification”. There is a major difference!
- Do you need material traceability? Nuclear people often require this. The material has to be traced from the original mill (melt and batch numbers) through manufacture and supplier to the end customer. It ‘s a costly procedure to provide all of the paper work to document all of this
- Maybe you need only the “physicals”. Things such as grade, tensile strength, coefficient of expansion, etc. This information is usually readily available and should not add to the cost of the product.
- Perhaps you need only a list of the ingredients. If you are purchasing stainless steel maybe you need only the percentage of chrome, nickel, sulfur, etc.
- Tolerances can be tricky. If you don’t need a tight tolerance, don’t ask for it. Tight tolerances always come at a high price because they require extra machining steps There is a big difference between ± 0.001″ and ± 0.0001″.
People that deal with elements such as carbon or ceramic often keep their process and tolerances a secret. They should not be reluctant to supply the “physicals”, but seldom will discuss their manufacturing techniques. It is these techniques that give them their competitive edge.
The product must meet a specification
- Is the specification for materials or design, there is a major difference! Many times there is no specification, but the engineer assumes there is one and asks for it any way. Questions like, “Does this seal have FDA. (Food and Drug Administration) approval? There are no FDA approved seal designs, and in most cases the materials are not specified.
- Some specifications are real. Meeting the API specification dictates a special mechanical seal gland that incorporates certain features and tolerances, along with fitting sizes. Do you really need that specification, or do you want the seal gland to perform the function? If you are working in the petroleum industry you will need a gland that meets the API specification, but outside of the petroleum industry you need only a gland that performs the function which is available at a much lower cost, and will fit process pumps without a costly modification.
- The ANSI, ISO, and DIN centrifugal pump specifications describe a pump with an impeller located too far from the bearings. If you want reliable mechanical seal performance you will not want a pump built to any of these standards, you will have to purchase a more expensive “heavy duty” design.
The only way to solve the problem with specifications is to give the supplier a copy of the specification you want him to meet whenever you buy what ever it is you’re going to purchase. Don’t ask him to meet the specification, give a copy of the specifications to him.
You have special knowledge about the fluids you are going to seal that is not generally known outside of the industry. This lack of knowledge on the part of the supplier can result in a premature failure with all of its associated problems and costs.
- Ask a mechanical seal supplier to recommend an O-ring for the dye used in the textile industry and he will recommend Viton®. What he doesn’t know is that a caustic “boil out” is common in the process, and Viton® will be destroyed during the “boil out” phase. Unfortunately the failure will occur five to ten days after the “boil out” so the connection between “cause and effect” is not always obvious.
- Kaoline is a product used in the paper and a few other industries. Does your seal supplier know that Kaoline is a very unique product that has abrasive solid particles less than one micron in size, that will penetrate between lapped seal faces and damage the sealing surfaces?
- Do you commonly clean or flush the process lines with caustic, steam or some other type of cleaner? The O-ring that your supplier will be choosing must be chemically compatible with this cleaner as well as the product you will be sealing.
- The paper industry also uses chemicals called black liquor, green liquor and white liquor. Depending upon the process these products could be either acidic or caustic. If your seal supplier makes the wrong choice you will have a seal failure within two weeks when the elastomer swells up and locks the seal. A more serious problem can occur if Reaction Bonded Silicone Carbide was chosen as a hard face. This material can break down in high pH liquids.
- Buying a new pump? What data did you give the supplier. Did you give him a print? The head and capacity? Maybe you just gave him the size of the old pump, or a diagram of the piping layout. Are any of these good enough?
- The piping is often different than that shown on the print. When lines are added the print is seldom changed.
- Did you supply a real system curve or just a simple single head/capacity point?
- Are you sure the existing pump is the correct size? If there is a mechanical seal installed you know the seal is experiencing premature failure (the sacrificial carbon is not wearing out). Maybe the shaft is deflecting because the pump is running off of its B.E.P. or cavitating. Most existing pumps were supplied oversized to compensate for the safety margins that were added by the people making the original calculations and recommendations.
Do you work in the marine industry? That industry is a lot different than the process industry.
- Alignment between the pump and driver is different because the ship’s hull flexes. Off shore platforms do the same thing making most alignment techniques ineffective.
- Salt water is a unique combination of high chlorides and low pH (8 to 9). This is a deadly combination for “crevice corrosion” of any metal that passivated by forming a protective oxide layer (ceramic) on the surface of the metal. Stainless steel, alloy 20 and titanium are a couple of examples of such metals. The corrosion takes place between close fitting parts and especially under O-rings and clamped gaskets unless these parts are lubricated with zinc oxide or a similar sacrificial anode material. Does your supplier understand this, or will he supply you with a standard O-ring lubricant and six months later you have the problem?
There is something unique about your plant or operation.
- High levels of Ozone will attack Buna N rubber compounds. These rubber materials are commonly used in the popular rubber bellows seal designs (Crane #1, #2 or #21). Some facilities have high levels of ozone in the shop atmosphere and the seal supplier might not know it.
- You may be using a special O-ring lubricant that is compatible with your product, but not the elastomer in the mechanical seal, valve, gauge, etc. If you are using anything other than silicone grease this could be a problem, so check with your supplier. As an example&emdash;petroleum products cannot be used on EPR O-rings. If you make this error the failure will usually occur within ten to fourteen days.
- Does your mixer alternate between pressure and vacuum? A conventional balanced seal is balanced in only one direction. You may need a two way balanced design. Many split seals also have a problem with alternating pressure direction.
My many years in this business has taught me that you only get the right answers when you ask the right questions. Recognize that most of us don’t know the right questions, so be sure you volunteer your inside information to save both yourself and the vendor the obvious problems that arise from his lack of knowledge about your product or process.
® DuPont Dow elastomer
- On February 18, 2018