Manual Gate Valves

In the last hundred years, valves have settled down into basic types such as gate valves, globe valves, check valves and others. This article will deal principally with the basic type called gate valves.

Gate valves are primarily for open and closed service, and are not particularly adapted for intermediate flow regulation or throttling service. Due to their low head loss, which is only slightly greater than the equivalent length of pipe, they are particularly adapted for those services for isolation of equipment, where for long periods of time they will remain in the open or closed position, and offer little or no obstruction to the flow of fluid in the pipe line.

Characteristics of this Type

Gate valves cover a wide range of sizes in common American practice - from 3/8 to 108 in., although in later years the 84 to 108 in. sizes have been supplemented by the butterfly valves, usually due to the space limitations under which they are installed.

Gate vales have been used successfully in temperature ranges of minus 300°F to 1800°F, and at pressures of from a few ounces of gas to thousands of pounds per square inch. Naturally the wide range of pressures and temperatures under which these valves are utilized necessitates a very wide range of materials of construction. Materials that are suitable at minus 300°F might not be suitable at 1800°F. At the present time gate valves are being manufactured in brass and bronze, several grades of gray iron, the newer ductile iron and the whole gamut of steel types, together with numerous special alloys for corrosion services.

The "trim" materials for the non-corrosive seats are being made from bronzes, stainless steels, Monel and hard facing alloys. Valve stems cover a wide range of materials based on the temperature and pressure at which the valve is to be used. In addition, at elevated temperatures, ingenious means must be provided to keep the valve spindle stiff in order to resist column action occasioned by the operation of the valve.

Originally, in this country, valves were made in the smaller sizes with screwed connections and in the larger sizes with flanged connections. With the development of welding techniques in the early 1930's, valves were made with welding ends to be welded to the pipe in which the flow is regulated. Later, other types were developed, namely, pressure seal joint, "O" ring joint, and so on.

In their simplest construction, gate valves have a valve stem, or spindle, which projects from the waterway to the outside atmosphere, for controlling the gate. This necessitates some method of retaining the fluid in the pipe line. Rope packing with a gland was the principal means of resisting this leakage. The gland is still the most widely used method. However, the materials used for the packing have been changed and improved. Today gate valves are made in some instances and for special applications with trick glands such as "O"rings or bellows seals; and, for atomic jobs, where any leakage would be disastrous, with canned or hermetically-sealed construction which eliminates the need for a gland.

There are a great many variations of gate valve designs. The principal one that comes to mind is the bonnet joint where two halves of the gate valve bodies are fastened together.

In the small bronze valves, common practice was to use screwed joints or union bonnet-type joints. However, as the valves became larger, or the pressures or temperatures were increased, other means had to be developed to retain the pressure. On the larger valves the joints were of the flanged type with many variations of gasket types and material. These proved satisfactory for a long period of time, but as pressures and temperatures were increased in central stations and oil refineries in the 20's, it became necessary to find a type of joint that would remain tight for longer periods.

The "Sargol" type (or combination joint), which is a flange joint with a seal weld, was at first thought to be the answer. With the development of better bolting materials and gasket materials, the early difficulties with flange joints were eliminated, by the development of the "Houston", or ring joint. Again the central stations did not remain idle but increased the pressures and temperatures up beyond the limit of bolting materials.

As a result, designers had to make a design to contain these elevated temperatures and pressures. In 1936 the first all-welded construction with a welded bonnet joint was brought out by The Chapman Valve Manufacturing Company. By 1946, the pressure-seal type joint, developed for feed water heaters and heat-exchangers in Europe, was applied to gate valve bonnets. High pressure valves for central stations are largely of the welded, or pressure-seal, bonnet type construction.

The advent of the atomic energy program, with its necessary extreme tightness of the bonnet joint and ready accessibility for taking apart, has forced the designer to reach back into the 1920's to the "Sargal-Sarlun" joint, with the added advantages of better materials, better welding techniques and better control of joint configuration.

 

Gate Valve History

In the early days of the gate valve, the gate was parallel-sided, and was pushed into place in the main pipe line, to shut off the flow by relying on the pressure on the upstream side. The pressure of the fluid on the upstream side was relied on to create the necessary force to seal off the valve. Elaborate methods were developed in subsequent years so that the gate was pushed tight against both faces by mechanical means, making the action of the valve independent of the pressure in the pipe line. This type of construction had the disadvantage of a large number of delicate parts which were subject to attack by the fluid in the pipe line.

In the 1860's, a man named Chapman, while chopping wood one day, looked at the shape of the axe head and invented the solid wedge type gate valve as we know it today. Later, as the tendency to combine the advantages of both the solid wedge and double disk gate valve was tried, a design was evolved known as the split wedge. This type of construction has the advantage, although more expensive than the solid wedge, of being able to adapt itself to small amounts of distortion occasioned by pipe line strain. It also offers the peculiar advantage for those special installations where it is desired to seal the space between the seats with steam or inert gases.

Lately, a new variation has come on the market, the so-called "flexible wedge", wherein the wedge is in one piece but cut out between the two seats in such a way as to provide a small degree of flexibility. It offers the added advantage of relieving the valve spindle from excess stress when it is necessary to close the gate valve when subjected to extremely high temperature. In this connection it might be interesting to note that the atomic energy field again has dipped back into the 1920's for a valve construction with parallel seats, and relying on the pressure in the pipe line to keep the valve tight. This has been possible because, although the pressures encountered with pressurized water reactors are high, the temperature is less than 600°F.

Due to the type of service in which gate valves are used, they often require little or no replacement of parts. In extreme service it may be necessary to replace seat rings, gate, and valve spindle. The latter usually has to be replaced because of abuse.

 

Valve Standards

Today's modern gate valves are made from very carefully controlled materials. The materials are usually specified to American Society of Testing Materials' specifications. Pressure-temperature ratings have been standardized by the American Standards Association (B16 standards). Dimensions have been standardized so that usually the valves from on manufacturer are interchangeable with another. In addition, where the standards associations have not seen fit to publish a standard, the Manufacturers Standardization Society of the Valve and Fittings Industry has published what they call Special Practices. Other standards are in the process of being developed for radio-graphic examination of valve parts for exacting services of nuclear and supercritical installations. Other special testing specifications are being prepared which undoubtedly will be adopted as standards by American Society of Mechanical Engineers.

 

Operation of Gate Valves

When we think of the method of operating gate valves we immediately think of a handwheel, as it is only necessary to turn the handwheel clockwise to close it, or counterclockwise to open it. However, the sizes of gate valves have been increasing, and pressures and temperatures have been increasing; and, as a result they far outstrip the strength of one man operating on the rim of the handwheel. A 10-in. boiler feed valve in a modern power plant quite often has a load on the valve stem equal to 25 tons or more. Obviously, without mechanical assistance, an operator is not going to be bale to turn the handwheel to operate the valve. In order to provide mechanical assistance for operating these valves it is necessary to calculate the operating effort required, which is usually assumed to be composed of three principal parts.

1. Pinch on the wedge, either occasioned by the thrust required to seat it, or by thermal contraction and subsequent cooling, after a high temperature valve is closed.

2. Sliding friction of the sliding gate over the seat, which is a function of the differential pressure across the valve and the coefficient of sliding friction of the seating faces.

3. Piston effect of the vale stem occasioned by the pressure inside the valve being higher than the atmospheric pressure. To this should be added stuffing box friction, occasioned by sliding the spindle through the tight packing of the stuffing box.

The usual method of calculating the thrust on the valve stem is to multiply the area of the gate ( in contact with the seat ring) by the difference between the upstream and downstream pressure in pounds per square inch. In the case of globe valves, where it is necessary that the disk be wedged into the seat, the effort would be just as great. In the cast of gate valves this would be multiplied by the coefficient of friction of dry metal seats sliding one over the other, depending on the seating material. This value varies between 0.2 and 0.3. For example, when 100 sq. in. of gate is subjected to 100,000 lb of pressure, the load on the gate would be 100,000 lb; and if the coefficient of friction of the seats was assumed to be 0.2 of the 100,000 lb, or 20,000 lb, or 10 tons. In the case of globe valves the load would be the full 100,000 lb.

Most power-operated or hand-operated gate valves employ a screwed valve spindle to control the movement of the gate. Using the screw jack formula, with an assumed valve spindle 1 7/8 in. diameter, with 1/3 in. pitch, 1/3 in. lead and 0.3 coefficient of friction, and ball- or roller-bearings under the yoke nut, the above example would result in a torque of 530 ft-lb. If the handwheel was assumed to be 2 ft. in diameter with a 1-ft. radius, a man would have to exert 530 lb at the 1-ft radius, which is beyond his strength. A man is assumed to be able to exert 40 to 100 lb on the rim of the handwheel. If the handwheel diameter was increased to bring the effort lower, the radius of the handwheel would have to be 5 ft. 3 in. which is seldom possible.

Motor-Operated Valves

In our example we have eliminated or neglected the effect of the wedge pinch, which is always a difficult matter to figure. Electric motors can overcome this by providing a mechanical hammerblow device to unwedge the valve; and, some hand-operated valves employ mechanical hammer blows. Split, or flexible, wedge valves are not subject to the same stresses, or wedge pinch, as solid wedge valves. We have neglected also the piston effect of the spindle, although this could add another few thousand pounds to the total operating effort. We have also overlooked stuffing box friction on the spindle, which could add a ton or more to the operating effort.

In considering this problem, the first thing that comes to mind is the use of gearing as a means of reducing the effort. There are a great many gear heads on the market today, most of which employ worm-gearing, due to the compactness of the gear ratio. Worm-gears are available with a gear ratio as high as 40 to 1. However, while we have reduced the effort at the rim of the handwheel, we have sacrificed speed. In the example mentioned, the valve would have operated in thirty turns of the handwheel. If we put in an 8 to 1 gear ratio, we have increased the number of turns by eight, or from 30 to 240 revolutions.

If the valve is to be operated once a year, and time is not a problem, a man or a relay of men could operate the valve. If the valve is to be opened and closed every day (or every few hours as in process work), we would be faced with a labor problem, and some other means would have to be employed. Electric motors are often used to drive the gearing for operating valves. This presents the problem of how to stop the motor at extremes of travel. There are two methods of doing this.

1. A positioning limit switch which, after a definite number of turns of operation, trips to deenergize the motor.

2. When the valve seats, the load on the motor builds up, and a sensitive device trips the motor from the line.

Actually, both methods have their disadvantages. As this is not an article on electrical valve operation, we do not wish to become involved in the relative merits of the two methods.

Substitution of pneumatic motors, for electric motors, or valve operators, is one alternative for taking the drudgery out of valve operation. Piston operators are also employed extensively on low-pressure valves, in waterworks systems.

The designer of a piping system would do well to take the time to familiarize himself with what is required to open or close the valves he has specified, taking care to specify which valves need gears, and which need electric motors.

In addition to the selection of the proper valve operator, the designer should pay careful attention to the physical placement of valves to avoid conflicts.