Manual Plug Valves

Because of its compactness and simplicity, the lubricated plug valve is gaining fast acceptance by power plant engineers. It is easy to operate, offers positive closure, and lends itself to automatic control - a popular trend in power plant operation.

Already well established in petroleum, chemical process and related fields, the lubricated plug valve now is proving its worth in power plants, within a wide range of temperatures and pressures. It can safely and efficiently handle gas and liquid fuel, boiler feedwater, condensate, and similar elements.

Several basic features make the lubricated plug valve ideal for power plant application. Unlike many common types of valves, it has no high projecting yokes or bonnets, no exposed threads, no underhanging body to waste vital space. In complex piping systems, valves may be easily operated by centralized control. And there's no danger of leaks, even in gas piping systems, because of the valve's unique method of sealing.

Valve Design

The lubricated plug valve is of the "block" or complete shut-off type. It may be used for throttling where accurate adjustment is not necessary. But the primary service is positive open-close operation.

Three basic parts comprise the lubricated plug valve: body, cover and plug. The plug is the only movable member. The valve is opened by rotation only, the plug turning on a lubricated seat. While the plug is available in cylindrical and tapered form, the latter is generally preferred, because the tapered rotary principle results in quick and simple valve operation. The conical shape of the plug combines the tight fit advantage of wedge action with the sliding action of rotary movement. Rotation eliminates the need for raising and lowering a movable valve member, and protects seating surfaces from exposure to corrosive elements.

A lubricated gland-joint shoulder is used, rather than conventional stuffing boxes which operate by gripping around a stem or rod. The shoulder joint is compact, produces a minimum of friction, and combines the functions of stem seal and thrust bearing. Application of the shoulder seal principle involves either the two-bolt cover (which combines cover and gland in one piece), screwed gland, or bolted gland. Thrust against the upper shoulder of the tapered plug is transmitted through a resilient cushion packing and a sealing diaphragm gasket. The sealing member is a thin flexible metal diaphragm.

The diaphragm assembly is clamped firmly together and does not rotate. As the valve is turned between open and closed positions, the ground surface at the top of the plug slides freely against the metal diaphragm. Leakage is prevented by the lubricated close fit between the two parts and the narrow area of contact through which the plug-sealing thrust is applied.

Pressure Lubrication

Pressure lubrication performs three important functions in this valve: 1 provides "jacking" action; 2, seals the valve; 3, minimizes friction.

Lubricant pressure exerts powerful jacking action against the smaller end of the valve plug, momentarily unseating it. This action overcomes any adhesion and, because of the taper, creates a small space for the confined lubricant film between the plug and body. As a result, no matter how long the valve remains closed, the plug may be unseated without using a hammer or wrench. Further, the jacking not only puts a valve into the operation after long disuse, but provides lubrication at the same time.

The usual means of obtaining lubricant pressure is an elongated screw, plug shank. Because of its small diameter and the use of a fine-pitched thread, the lubricant screw develops ample pressure on the lubricant, yet turns freely. These screws are often provided drilled and fitted with a little giant "button-head" fitting for grease-gun lubrication.

Lubricant is distributed between the plug and valve body by means of lubricant grooves, divided between the body and plug. The grooves are so located that when the valve is in the closed position, valve ports are completely sealed by a lubricant film between the ground seating surfaces. Placement of grooves is such that groove sections exposed during operation are disconnected from the lubricant supply before they are brought in contact with the line-fluid. The grooves are automatically reconnected when the plug is brought to the fully open or closed position. Thus the valve can be partly opened without blowing out the lubricant.

One of the outstanding features of the plug valve is its quick opening-and-closing operation. A quarter turn fully opens or closes the valve. With the help of a stop collar on the valve, the operator does not have to rely on the feel of a wheel's resistance to tell if the valve is closed. Valves are wrench, or gear, operated.

Most all plug valves have a flat or a square wrench head on the plug stem and a stop-and-indicating collar which also serves as a weather shield for the stem clearance space.

Because of the tapered plug and lubricant sealed design of these valves, they are not restricted to one-way flow, i.e., the valve can be installed in either direction because it holds pressure in either direction. Flow can be reversed any time without the danger of leakage or jamming.

This feature means a constantly renewable valve seat. Instead of expensive repairs on seat and closing members, simple lubrication renews the sealing surfaces without necessity of dismantling the valve or even removing it from the line.

A common hazard to the life of valve seats is the abrasion caused by dust, dirt, and debris in the line. This hazard is entirely eliminated in the lubricated plug valve if proper gland adjustment is maintained. Critical seat areas are fully lubricant sealed, and the lubricant is under pressure so that abrasive material cannot intrude. Any dirt or material that may reach the plug is scraped off when plug is turned back to open position.

Regardless of line pressure, the plug valve can be relubricated as effectively as with no pressure. Lubricant pressure builds hydraulic pressure within the valve to the required degree. In this way the plug can always be effectively lubricated and the jacking chamber under the plug adequately pressurized to permit easy turning. As a result, even if a lubricated plug valve is unopened for extended periods, it can be easily opened without the difficulties of galling or freezing.

Lubricated plug valves are available in a variety of patterns, metals, and sizes, depending on their application. To handle the wide range of temperatures, pressures, and services, valves are made from a number of metals and alloys including bronze, carbon steel, alloy steel, semi-steel, and stainless steel - hard faced or chrome plated. They are obtainable in sizes to 30 in., and to 15,000 psi.

Valves can be equipped with every type of standard or special flanges, including Dresser ends. For jacketed service, valves are available with complete flange-to-flange jacketing in standard sizes. Plug valves are also made in patterns interchangeable with standard gate valves.

Another unique characteristic of plug valves is the ease with which they can be adapted for multi-port construction. In the three-way form a single L-shaped passage connects any of two ports; with a T-shaped opening any of three ports are connected; and the four-way type has two cut-outs on opposite sides which connect any two adjacent parts of four ports with a quarter turn.

An important adaptation in a number of plants is use of the valve in mechanical blinding. Two straight-way lubricated plug valves may be butted together in series and operated in unison through a common linkage. A bleeder valve located between them is operated through the same linkage in such a way that when the two block valves are closed the bleeder is automatically opened. This arrangement avoids drainage losses and prevents pressure build-up and the resulting hazards.

At gas-fired power plants, a special plug valve is available for positive prevention of fuel explosions. Approved by Factory Mutual Laboratories, the method assures closure of all individual burner cocks before the main burner gas safety shutoff valve can be opened. The valve is similar to a conventional lubricating unit, except that it incorporates two side outlets connected in series with copper tubing to adjacent valves so that when the gas passages of all valves are closed a new continuous passageway is provided through all closed valves in the system.

Valve Accessories

A wide variety of devices is available for special operation of the lubricated plug valve. Locking accessories, for instance, are obtainable for sealing a valve in either open or closed position, preventing removal of the valve or tampering with its parts. For hard-to-get-at places, chain wheels, double end chain wrenches, and a variety of extensions are available.

Simple quarter-turn operation makes the lubricated plug valve ideal for an automated power plant. Remote power operation extends the sphere of valve control because line flow can be manipulated from a centralized panel board. It can save many man-hours in normal operations, and preserve a large investment in case of emergency.

Valves may be operated pneumatically, hydraulically or electrically, in connection with automatic time clock, level, temperature, flow, or differential control systems. In all such systems, valve controls may be interconnected to give "chain reaction" control for sequence operations of all types. Power is applied by various types of operators, which can be either semi-automatic or fully automatic. And sometimes the valve may be powered by fluid energy drawn directly from the line.

Because of the wide range of conditions for which plug valves are designed, a number of lubricants are available. And the use of the proper lubricant is important because the valve structure and plastic sealing film are an integral unit, each component dependent on the other for best performance. Obtainable in sticks, tubes, and bulk form, lubricant can be added to the valve by hand, hand gun, or bucket pump. It takes a minimum of time, yet a simple program of periodic lubrication will keep valves in service for the life of the piping.

To simplify lubricant selection, valve manufacturers continue to reduce the number of choices by developing basic, multi-purpose lubricants. An idea of their progress may be gained by considering the properties of the recently developed 555, which replaced four other lubricants in the Rockwell-Nordstrom line. The new lubricant has an efficient temperature range of 40°F below zero t 500 above and is recommended for use with a variety of piping elements.

Maintenance Tips

To help operators get maximum life from lubricated plug valves, the following tips are offered:

Plug adjustments should be snug tight enough to keep the plug from becoming unseated, but not to a degree requiring excessive force to operate the valve.

Lubrication should be periodic, systematic, and with the proper grade of lubricant.

If the vale system is filled, fresh supply of lubricant can easily be forced between the seating surfaces by giving the lubricant screw a few turns. Depletion of lubricant in the system will require the addition of several sticks before lubricant can be forced into the seat. In this instance, lubricant should be added until resistance is felt in turning the screw.

Turn the plug slightly when lubricating to assist in the distribution of lubricant and to determine if the adjustment is correct. If the valve is lubricated with the adjustment too loose, it can be corrected by alternately tightening the adjustment and turning the plug. This will work out excess lubricant and permit the plug to return to its proper position in the seat.

At the time of adding new lubricant, care should be taken to prevent any solid foreign material from entering into the lubricant space of the valve.

Lubricated plug valves are usually worth reclaiming if their condition is such as to require only cleaning, inspection and reassembly with new packing and gaskets. Sometimes, a light re-lapping operation may be required to eliminate scaly deposits and superficial scoring or pitting. Judgment must be exercised in cases where the valve body needs reboring and fitting with an oversize plug, or building up of surfaces by welding.

This type of work is not economically justified for low-and medium-pressure valves. As a general rule, complete rebuilding of 4-in., and smaller 175 lb WOG; or 2-in., and smaller 200, 400, 500, and 800 lb WOG semi-steel valves should not be undertaken. A decision should be based on the actual cost in equipment, labor and overhead.

Valves with a heavy accumulation of dirt, paint and other foreign matter should be cleaned before disassembly. After complete disassembly of the valve, parts should be cleaned by using any suitable alkaline or solvent solution. Corroded areas may require wire brushing or abrasive blasting.

Repair procedures are determined by the extent and depth of blemishes in seating surfaces of both plug and body. Tapered seats of both body and plug should be carefully examined for corrosion, pitting, scoring, galling or erosion. For plugs with integral shanks, the machined shank diameter and the adjacent shoulder on the plug should be similarly examined.

Future Developments

Manufacturers continue to work toward increasing the scope of the lubricated plug valve. Not only are new alloys under test, but improved coatings may soon be available to help extend the life of the plug valve.

Probably the greatest challenge to today's manufacturer is the field of nuclear power plants. Radiation introduces unique factors in the design of piping systems, especially in the primary loops. Here, new types of corrosion resistant cladding will have to be developed before the plug valve can be applied. But outside the primary loops, piping in a nuclear power plant probably will not differ much from that in conventional plants. And the plug valve's inherent characteristics of safety, simplicity, and positive closure should make it an ideal component.