Secondary Seals Matter

Most pumps go into the shop, not because they are broken, but because they are leaking. Waiting for breakfast recently in a South American hotel, I perused the morning newspaper. The headline read, "Ammonia Blast Burns Dairy Worker!" The article started, "One man was hospitalized with second- and third-degree burns and four women were injured Monday morning when a refrigeration system o-ring burst, spewing an ammonia cloud into the margarine production building at Sol Refining Co."

A milk processing plant uses many pumps and compressors. Liquid dairy products are pumped through the plant. Air is injected into ice cream, sherbet, yogurt, butter, and margarine; it expands the volume and makes us think we’re getting more for our money. Also, air is used for the packaging and boxing line. Natural gas is used for cooking and pasteurizing milk products. Ammonia is compressed and pumped as a refrigerant.

To conserve parts, many factories and process plants will standardize on one or two models of a pump or control device that can be used throughout the plant. This philosophy is conducive to unexplained mysterious seal failure and leakage.

A flowmeter doesn’t care if it’s measuring the flow of liquid ammonia, fuel oil, or milk. But the o-rings used as secondary seals care. A mechanic had installed the wrong replacement seal kit into a control valve.

At the plant accident detailed above, the rubber seals were not compatible with the ammonia, leading to its failure, the ammonia cloud, and injuries.

Most square and rectangular shapes in industry require some kind of gasket material cut to seal the odd shape. Most round forms requiring a seal (shafts, sleeves, mechanical seals, tubing, housings, threaded joints, holes, and bores) use the lowly and shy o-ring as a secondary seal.

We have o-ring seals all over the house and throughout our lives. A standard refrigerator has about 200 o-rings in it. A passenger jet could have over 50,000 o-ring seals on it.

About half of all pumps in the maintenance shop today were pulled out of service because they were leaking or wouldn’t hold pressure. This is most likely a leaking gasket or o-ring. The o-ring is the rubber component of most pump seals and most instrumentation fittings. The o-ring controls the temperature, pressure, and chemical limits of the device.

The difference between a flowmeter in alcohol service and a flowmeter in condensate service is the o-ring. It is not the stainless steel, or the strict tolerances, or the flange bolts. The difference between a mechanical seal in ammonia service and a mechanical seal in propane service is the o-ring.

The people who assemble instrumentation devices and parts for the chemical process industries install o-rings that are adequate to perform the static pressure and vacuum test on the part at the factory, which is normally done with water or air. The ultimate user must verify that the factory-installed o-rings are correct for the pumped product application (take into account temperature, pressure, chemical compatibility, and shelf life).

A pressure gauge may be factory tested and calibrated on air or water. You don’t have free reins to open the box and install that gauge into ammonia, propane, or MEA (monoethanolamine) service without verifying the secondary seals.

If the seals are inadequate, they must be changed before installing the device into the process piping system. Because most o-rings are black and appear the same, it may be necessary to use an identification tool, or purchase individually packaged and labeled o-rings to identify the o-ring rubber compound. Remember that once the package is opened and stapled shut, the control is lost.

There are many rubber compounds used in industry as o-ring secondary seals. Some elastomer compounds are highly specialized, with applications only in certain industries. Here are four rubber compounds, which find broad popularity in almost any production plant.

Fluorocarbon ("Viton" is Dupont’s trade name) 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 from -15 F to 400 F (-25 C to 205 C).

Perfluoroelastomer ("Kalrez" by Dupont, "Parafluor" by Parker, "Chemraz" by Greene Tweed) is a rubber compound compatible with most organic and inorganic liquids and gases and aggressive chemicals. This material finds popularity in instrumentation devices used in chemical processing and pharmaceutical plants, and wherever the temperature of the application demands. It is recommended for temperatures ranging from about -20 F to 550 F (-30 C to 260 C). (The purchasing agent may fall out of his chair when he sees the quote. But they work where they are needed.)

Nitrile (Buna-N) is a rubber compound popular in most sinks, drains, bathrooms, and gardens. It’s a basic plumbers o-ring seal and handles most household liquids and chemicals (think of laundry soap, sour milk, cooking oil, bleach, shampoo, and fabric softener). Because water is the most popular liquid in industry, this elastomer may be the single most popular o-ring secondary seal in the world. Its service range is from -30 F to 225 F (-34 C to 115 C).

Ethylene Propylene (EP, EPDM) is an o-ring rubber compound that is compatible with most water-based industrial chemicals. It is good with caustic soda, detergents, water treatment chemicals, steam, condensate, treated water and wastewater. EP is the standard seal for food process liquids like milk, beer, juice, sauces, and soups. EP is good from about -70 F to 300 F (-57 C to 150 C).EP rubber is a petroleum-based compound. For this reason it should never be exposed to petroleum-based chemicals. It will dissolve.

When I was a young steel mill mechanic in Birmingham, Ala. back in the 1960s, I installed many mechanical seals with EP o rings onto some treated water pumps for the hydrostatic pipe-testing tank. My foreman had told me to protect the o-rings by greasing them before sliding the seal over the pump shaft. We rebuilt the pumps and changed seals every week. Our seal salesman loved us. (He also never coached us on proper o-ring selection.)

One day the grease bucket disappeared. I had to spit on the o-ring seals to lubricate them at installation. The weekly seal failures stopped. You DO NOT put petroleum-based grease onto an EP o-ring to help it slide over the shaft. (Put this into your CHEAT SHEETS!!) (And silicone grease is better than spit.)

Let’s consider an industrial boiler. The wrong o-ring may not be catastrophic, but it’s called a "leaking boiler". You may need three or even all four o-ring compounds 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 (buna) rubber o-ring seals.

Next, the raw water must be treated before it can be pumped into the boiler. Chemicals are mixed with the raw water to control the pH, and remove oxygen and minerals from the water. Treated boiler water is called "makeup water". The treatment chemicals and the makeup water will need Ethylene Propylene o-rings on all instrumentation, gauges, connections, fittings, valves, flanges, and mechanical seals.

The DA tank, feed water pump, boiler water, steam, and condensate also require EP seals. If the boiler is a high-pressure boiler, the boiler’s discharge valves and instrumentation fittings may need perfluoroelastomer o-rings for temperatures above 300 F. 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 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, fuel pump, valves, instrumentation, and fittings. Who would have thought that an industrial boiler would need up to four different o-ring compounds just to heat and seal some water?

Remember, 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 $5,000 instrumentation devices and fittings are tossed into the recycle bin or thrown away because of an 11¢ o-ring. We’ll cover this and some other important points at my upcoming workshop, Oct. 1-2 at ISA Expo 2007 in Houston, TX. I encourage you to attend this training event, and bring your foreman.

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at [email protected] or 615 361-7295.

www.bachusinc.com