Oring The elastomer failure

About half of all pumps in the shop today were pulled out of service because they were leaking or wouldn't hold pressure or pump. This is most likely a leaking o-ring. The o-ring is the rubber component of most mechanical seals. The o-ring controls the temperature, pressure, and chemical resistance of the mechanical seal (Figure 14-1).

The difference between a mechanical seal in a pump in alcohol service and a pump in steam service is the o-ring. It is not the stainless steel, or the ceramic face of the seal. The difference between a mechanical seal in

Figure 14-1

ammonia service, and a mechanical seal in propane service, is die o-ring. The people who assemble seals install o-rings that are adequate to perform the static pressure and vacuum test, which is normally done with water or air. The user must verify that the seal elastomers, the o-rings, installed in the factory are adequate for the service application (temperature, pressure, and chemically compatible). If they are inadequate, they must be exchanged for the correct o-ring rubber compound before the installation. It may be necessary to use a tool, like the following, to identify the o-ring elastomeric compound (Figure 14-2).

Common o-ring compounds used in mechanical seals

There are many rubber compounds used in industry as o-ring secondary seals. Some elastomcric compounds are only found in one or two scaling applications in one industry. We have listed the following four compounds, which find broad popularity in almost any production plant.

1. Fluorocarbon (Viton®) is a rubber compound that is compatible (meaning it resists without degradation) with most petroleum based liquids and gases (propane, gasoline, crude oil), some acids and other chemicals. It is used extensively in the petroleum refining and petrochemical industry. Its temperature range is good from -15° F to +400° F. (-25° C to +205° C).

2. Perfluorocarbon (Kalrez®, Parofluor®, Chemraz®) is a rubber compound compatible with most organic and inorganic liquids and gases and aggressive chemicals. This material finds popularity in chemical processing and pharmaceutical plants, and wherever the temperature of the application demands. Its service range is from about -20° F to +500° F. (-30° C to +260° C).

3. Nitrile (Buna-N) is a rubber compound popular in most household plumbing applications. It's a basic plumbers o-ring seal, and handles most household liquids and chemicals. Because industry pumps so much water, this elastomer may be the single most popular o-ring secondary seal in the world. Its service range is from -30° F to +250° F (-34° C. to +120 C).

4. Ethylene Propylene (EP, EPDM) is an o-ring rubber compound that is compatible with most water-based chemicals. It is good with caustic soda, detergents, water treatment chemicals, steam, and wastewater and with food processes like milk, beer, and soups. EP rubber compound is petroleum based and for this reason it should never come into contact with petroleum based chemicals.

It will dissolve. Its service range is from -70° F to +300° F (-57° C to +150° C).

How many different o-rings to heat some water?

Let's consider an industrial boiler. You may need at least three of these previous mentioned o-rings just to prevent leaks and drips in a simple hydronic or steam boiler. Raw water comes into the boiler room with pipes, gauges, valves and instrumentation. All these fittings would probably use Nitrile rubber o-ring seals to give long-term leak free service.

Next, the raw water must be treated before it can be pumped into the boiler. Treating the boiler water does three things. First it controls the pH so that the boiler tubes won't corrode. Next the treatment process removes oxygen, which prevents internal boiler wetted parts from rusting. Third, the treatment process removes minerals from the water so that mineral scale won't form on the boiler tubes, insulating them, and causing the boiler to lose efficiency. After raw water has been treated with chemicals to scavenge the oxygen, remove or neutralize minerals, and control the pH, the raw water becomes make-up water. The treatment chemicals and the treated water will need Ethylene Propylene o-rings on the mechanical seals, instrumentation, valves, connections and fittings.

If the boiler is a high-pressure boiler, the boiler's discharge valves, and instrumentation fittings may need perfluorocarbon o-rings for temperatures above 300 degrees. The high-pressure boiler feed water pump may need these high temperature o-rings in the mechanical seals because of the high frictional heat generated by the seal faces. If the DA (deaerator) tank is sealed and pressurized to hold the hot water from flashing, it may need these high temperature o-rings.

If the boiler burns propane, natural gas, or fuel oil, then you'll need fluorocarbon o-rings on your fuel lines, valves, instrumentation and fittings.

Who would have thought that an industrial boiler would need up to four different o-ring compounds just to heat some water?

When an o-ring comes out of service, it should be inspected for signs of damage and degradation. These could be:

Chemical attack

Because the o-ring comes into contact with the fluid, the o-ring's rubber compound must be chemically compatible with the fluid. Chemically compatible means that the o-ring will resist the chemical without degradation. If the o-ring is not chemically compatible with the pumped liquid, it may swell, harden, dry and crack, soften, or even dissolve depending on the nature of the chemical attack. The surface of the o-ring may form blisters, scale, or form fissures and cracks. The cause of these symptoms generally is chemical attack. The attack may come from the pumped liquid, or from the barrier fluid or external flush in the case of installed environmental controls. Environmental controls are discussed later in this chapter. Too much heat in the system may present the same evidence. You must be familiar with the different o-ring compounds used as secondary seals in mechanical seals, and all instrumentation, connections and fittings. You must know the temperature limits (upper and lower) and the chemicals they are compatible with.

Figure 14-3 Chemical attack

Figure 14-4 Extrusion

Extrusion

Extrusion is deformation under pressure. An o-ring extrudes when the pressure is too high. Maybe the o-ring needs back-up rings to tighten tolerances. Maybe the design of the o-ring groove is inadequate. Some o-ring compounds get softer as the temperature rises so temperature-linked-to-pressure is also a factor to consider. An o-ring also may soften if under chemical attack. This should also be checked as a possible source of the extrusion.

Split, cracked or hard o-ring

Although an o-ring softens with temperature, too much heat will harden the o-ring. Too much heat is the usual cause, but chemical attack may produce the same result. The o-ring must deal with the temperature of the application and also from the heat generated between the mechanical seal faces. Many mechanical seal designs place the o-ring where it receives a lot of heat. A good idea is to find a mechanical seal with the o-rings and other secondary elastomer seals placed away from the heat of the faces.

Compression set

This is a good indicator that the O-ring was exposed to too much heat. Compression setting means that the round cross-sectioned o-ring came out of its groove with a squared cross section. The heat caused the o-ring to re-cure in the groove, taking the shape of the groove.

Figure 14-6 Compression set

Figure 14-5 Split, cracked or hard o-ring

Figure 14-6 Compression set

Ozone attack

O-rings, cspccially Buna-N (Nitrile compound), should be stored away from fluorescent lighting and electric motors. These arc sources of ozone. Ozone causes a general degradation of these elastomers.

Figure 14-7 Nicks and cuts

Nicks and cuts

This failure is mostly evident on start-up as immediate leakage. Extreme care should be taken to prevent damaging the o-rings as the mechanical seal is slid down the shaft at installation. The o-ring grips the shaft to withstand the seal's maximum pressure rating. The o-ring must slide over the impeller threads, key way grooves, steps on the shaft, and marks made by previous setscrews. O-rings damage easily.

AUTHOR'S NOTE

Most pumps don't come out of service because they break. Most pumps go into the shop because they're leaking. Every day all over the world, too many $2,000.00 mechanical seals are thrown into the recycle bin or thrown away because of an 11t o-ring.

0 0

Post a comment