Most valve professionals take more notice of the oil or fluid flowing through the valve than the oil that lubricates the valve’s mechanical parts. That should not be the case. Virtually every valve with moving metallic parts needs lubrication, and some valves such as lubricated plug valves simply cannot function without it.
Lubricants serve four main purposes in valves: 1) to grease metallic mechanical joints such as those found in stem to bushing and gear interfaces; 2) as a hydraulically activated surface coating to enable lubricated plug operation as well as allow some gate and ball valves to operate smoothly; 3) as an injectable, emergency sealant between disc or ball and valve seats; 4) as a coating to prevent corrosion or oxidation. Sometimes the corrosion prevention function is combined with a cavity filler function as well.
On most linear valves, the primary lubrication location is the stem bushing. This interface between a threaded stem and a similarly threaded bushing requires lubrication to keep it operating freely. Most manufacturers use a general-purpose grease for the job. In larger valves, there is usually a grease fitting to enable post-installation relubrication.
THE RIGHT SLICK
When people think about oil, they picture the stuff that goes into car engines or the little tube that says “Three in One.” But lubricants come in many types, chemistries and viscosities. From alcohol-thin oil to the thickest, stickiest grease, they all serve important purposes in operating valves.
Oils, which are also the base ingredient for most grease compounds, come in many forms. They can be petroleum-, animal- or plant based, or they can be synthetic creations. To make a grease, the liquid base material is combined with a suitable thickener, called a “soap,” to make the grease thick and sticky (or as a non-mechanically-inclined squeamish person might say—“thick and icky.”)
The viscosity of a grease is measured on a linear scale developed by the National Lubricating Grease Institute (NLGI). The NLGI scale runs from 000 (less viscous) to 6 (thickest). For valve applications, the greases used are usually in the medium viscosity range of 1-2.
Grease descriptions also often contain the prefix AP, MP or EP. These acronyms have important meanings:
- AP stands for “all-purpose” grease. These thick lubricants provide good thermal and structural stability. They also offer a high degree of corrosion and rust resistance. AP greases are often used on cars and trucks and in some less-demanding valve applications.
- MP, or “multi-purpose” grease is an NLGI number 2 grease also popular in the automotive industry as well as in light industrial and agricultural applications.
- EP, or “extreme pressure” grease is the heartiest of the three types. Under high pressure or shock loading, normal grease can be compressed to the extent that the protective film breaks down and the protected parts can come into physical contact, causing friction and wear. EP grease contains additives such as graphite or molybdenum disulfide to protect the grease under heavy loadings. EP greases also contain solids that bond to the surface of the metal, adding a sub-layer that prevents metal-to-metal contact. Valve gear boxes, both manual and powered, require a hearty grease to keep the gears meshing smoothly and inhibit corrosion in the gearbox. EP NLGI #1 or #2 grease is a popular choice for these valve applications.
Most valve lubricants are petroleum based. However, some applications, such as chlorine and oxygen service, require synthetic compounds. These synthetic compounds are often polytetrafluoroethylene (PTFE) based. Valves in oxygen service cannot tolerate petroleum-based lubricants of any kind because of the possibility of ignition. Other services such as chlor-alkali also require specific lubricants.
Valves used in high-temperature applications call for specially formulated high-temperature greases. These greases usually have a petroleum base that exists to spread additives such as graphite or lithium onto the metal parts. As the temperature rises and the petroleum products evaporate, the graphite or lithium coating remains, protecting the metallic surfaces from damage.
Injectable lubricants are popular in valves used in the midstream segment of the oil and gas industry. These applications, such as pipelines and gathering lines, are under the realm of the American Petroleum Institute (API) specification 6D, Pipeline Valves. Most 6D gate valves have grease fittings that allow a lubricant to be injected in the seat area to seal leaks or otherwise improve the valve’s shutoff characteristics. API 6D ball valves are also equipped with lubrication ports or fittings.
Injectable lubricants are usually pumped into the valve through use of a grease fitting equipped with ball check valves. The grease is pumped into the seat area then squirts out and spreads between the seat and closure member, creating a thin, leak-sealing film. Since most of these lubricants are petroleum-based, their use is limited to valves in hydrocarbon service. However, a few injectable lubricants are specifically formulated for oxygen, chlorine and similar non-hydrocarbon applications.
Lubricated plug valves must have lubrication to operate. When not moving, the metallic plug sits tightly against the metal seat in the valve, with only a thin film of lubricant between the seats and the plug. However, when the valve must operate, a squirt of very-high-pressure lubricant causes the plug to move slightly off the seat and easily rotate 90 degrees.
Like injectable pipeline valve greases, injectable lubricated plug valve grease compounds come in many flavors to serve a variety of different applications. In API 599, the metal plug valve standard, paragraph 6.1.5 details the requirements for lubricated plug valve grease:
“Unless otherwise specified in the purchase order, lubricated plug valves shall be furnished with hydrocarbon resistant lubricating sealant that has a temperature range from -20°F (-23°C) to 225°F (107°C). This sealant shall have both the proper plasticity for tight sealing and the lubricity for ease of operation.”
While all lubrication is important, for some applications, the use of lubrication in valves is strictly detailed and controlled. In API 598, the primary valve testing standard today, the use of lubricants in aiding sealability is limited. The wording in paragraph 6.4.1 states:
“If necessary to prevent galling (during the closure test procedure), the sealing surfaces may be coated with a film of oil that is not heavier than kerosene.”
Valve lubrication is vital today for effective and efficient valve operation. Long gone are the days when a few drops of 30-weight motor oil in a squirt-can would keep valves functioning. Today’s valve lubrication needs require specific compounds designed to meet the broad range of different valve applications. That means the squirt-can should remain on the shelf.