Manual Valve Operators

One important decision in designing of modern high-pressure power plants is the type of operators to be used on the large, high-pressure valves involved. Let's define "large" high-pressure valves as any over 10-in. at 600 psi, 8-in. at 1500 psi, or 6-in. at 2500 psi. In many cases the valve operator, plus its power and indicating connections, may cost more than the basic valve itself. Consequently, it is well worth the time to study actual requirements and to consider the many types of valve operators available.

The basic requirement is tied up in the operating characteristics of gate and globe valves. Almost without exception, such large valves are operated by a valve stem extending through a packing chamber to the closure member. The valve stem may travel a distance equal to the diameter of the pipe down to a distance 25 percent the diameter of the pipe. Thus, closing such a valve is equivalent to moving a piston, whose diameter is equal to the valve stem diameter, through a high-pressure chamber for a distance equal to total valve travel. In some modern piping systems, this can result in a considerable force requirement.

At the end of this travel, the closing force must be increased enough to create a tight seal on the valve seat. In typical large, high-pressure power plant valves, these closing forces may be as high as 25,000 lbs exerted through a distance of 10 in. just to push the stem into the valve, plus 250,000 lbs exerted through a distance of 1/10 in. to create a tight seal. Forces of this order acting through such distances represent a large amount of work.

About 1 hp would be required to close such a valve in one minute, if we had a perfectly efficient operating device. But, the devices that can generate such large forces are relatively inefficient, so the actual power requirements of operators for large valves are much greater than 1 hp.

 

Forces Undergo Transition

Because the greatest force is required for only a short distance near the point of seating, and because a lesser force is required for the longer distance between open and close, valve designers have sought many forms of valve operators to take advantage of the two force conditions. For example, typical gate valves almost always call for total stem travel equal to something a little more than the inside diameter of the pipeline. During this period of travel, whether opening or closing the valve, the total force on the stem is equal to pressure (inside the valve) times area of stem.

Now, the typical stem might have a cross section area of 7 sq.in. and might be acted upon by a pressure of 2500 psi. An outward thrust of 18,500 lbs would act on the stem at all times. To this we add the friction load for the valve packing...which frequently is figured as 10 percent of the stem load. So, we add another 2000 lbs to the 18,500 lbs above.

Actual seating of a wedge gate in its seat might seem rather difficult to calculate, at first glance. On the other hand, pulling the wedge off of its seat is perhaps a clearer thing to visualize. Under extreme conditions, the valve gate rests against the downstream seat face with a maximum of full line pressure on the upstream side and zero pressure on the downstream side. So, one of the forces holding the wedge against the seat is line pressure times seat area. The force to slide the gate off the seat is then line pressure times seat area times coefficient of friction of the sliding surfaces. A reasonably conservative figure for the friction coefficient on the internal metallic surfaces of a large, high-pressure valve might be 0.33. Operating force for pulling the valve open, on the basis of this calculation, then would be one third the line pressure differential across the wedge times the seat area.

If the gate were wedged between two seats, considerably higher force might be required to pull it free. Wedging action could be generated by initial closing, and/or by thermal expansion or contraction that took place after the valve had been closed. The two seats could have contracted against the faces of the gate wedge. The valve stem might have contracted less than the bonnet and yoke assembly. Hence cooling of the valve might have forced the gate into tighter engagement between the two seats. Forces of this nature are very difficult to calculate. Valve designers generally rely on operating experience to judge the magnitude of such forces. But they can be expressed generally as some fraction of operating differential pressure times valve seat area.

In securing tight closing of a wedge gate valve, total closing force requirement is often figured on the basis of differential pressure times seat area times friction coefficient, plus a 10 per cent additional load to provide a moderate amount of wedging between the two seat faces, to develop the maximum seat tightness.

Application of these formulae generally results in a force for seating and unseating gate valves that acts through a distance of about 1/4 to 1/2 in. This is roughly seven times the force for operating from closed to wide open. The transition from high force to lower force takes place rather abruptly near the fully closed position because line fluid forces have relatively little effect on the valve gate once it is slightly open.

 

Globes Require Higher Force

Globe valve operating requirements are much like gate valve requirements. Globe valves, however, have shorter total lift and require a greater seating force. Ordinary globe valves may lift only 25 per cent of the valve pipeline diameter, although specially designed low-pressure-drop valves may have stem travel as high as 75 or even 100 per cent of line diameter. Globe valves seat by setting the disc directly into the seat port, in opposition to the line pressure. Hence, the seating force required is pressure differential across the valve times full area of the seat port, without application of the friction coefficient noted for the gate valve. An allowance of 10 percent is added for tightness.

If the higher pressure is under the valve disc, the closing force is required as a compressive load in the valve stem to resist the pressure on the disc; this force is a tensile load on the valve stem, to lift the disc off the seat, when opening the valve.

As in the case of the gate valve, seat forces in a globe valve dissipate rather quickly upon opening. So the high force requirement lasts only through limited distance, say 1/4 or 1/2 in. Regular stem force is needed for 90 percent or more of the total stem travel. In a typical case, the globe valve seating force is 10 times the force required for moving from open to close.

 

Differential Is a Variable

Quite a distinction must be made between valve opening and closing forces, and valve seating forces. Because they differ by as much as 10 to 1, the distinction between these forces is important. In actual practice, however, the seating load itself often varies widely. In simple terms, the seat load required to close a valve is the full pressure differential as applied across the seat area. A conservative assumption is that the full internal pressure of the line is applied to one side of the seat and atmospheric pressure to the other. In certain valve operations, though, this is almost never the case. Many recirculating lines develop high pressure in the line, but the differential pressure across the circulating pump is low. The valve might close off the low-pressure across the circulating pump, and not close or open against the full line pressure, measured with respect to atmospheric pressure.

Less than full line pressure develops, also, where large valves are equipped with bypass lines. In many cases, the large valve is not operated until the bypass line has been opened, to hold down the differential across the large valve.

Boiler installations, in particular, may present pressures across a valve higher than the full design pressure. Boiler and piping code practices call for testing lines at 1 1/2 times working pressure. A valve may be used to isolate a section of the boiler or line for developing this test pressure. The valve closing mechanism must be 50 per cent stronger than might be expected under normal conditions.

 

Check Valves Need Less Force

Of particular interest in many power plant operations is the case of the stop-check valve. A free-floating disc in the valve has a valve stem for securing the disc to the seat. If the ordinary high-pressure differentials (created during closing) come from under the valve disc, the valve stem must transmit the same forces as for a globe valve. In many applications, though, the differential tends to be on top of the valve disc. The valve stem is used really as a securing device to hold the disc in place to avoid any accidental opening. Maximum seating load on the valve stem might be only three times the load required for pushing the stem from open to close. The valve operator requirement is somewhat different than for the standard globe or gate valve.

Practically all the valves under discussion are operated by a stem thread acting through a bronze nut. A few valves of very special design and application occasionally are operated by direct-acting hydraulic cylinders. The cylinder idea is always intriguing because of the low friction loss in such devices and the ability to multiply force greatly. But operating pressure must be continually maintained to hold the valve in any one position. Furthermore, failure of the hydraulic system would permit the valve to fly open. These disadvantages prevent any widespread use of direct cylinder operation.

The nut-and-screw-thread device is a relatively high-friction unit. Full load is transmitted through the stem threads into the valve operating nut. Appreciable friction on the thread surfaces calls for extra effort over and above that already established for the valve. The force developed for such screw thread construction can conveniently be calculated by means of a formula.

This formula, in most practical valve constructions, shows that the operating torque in ft lbs on the valve nut unit is about 1/100 of the pounds force developed in the stem.

 

Thread Friction Is Big Factor

With a rotating stem, the nut is fixed. Total friction is a combination of the thread friction, friction between valve disc and the end of the stem, and packing friction. In the case of a non-rotating stem, the nut is generally mounted on ball or roller bearings. Total friction is a combination of the thread friction, ball or roller bearing friction, and packing friction. In either case, thread friction accounts for more than 75 per cent of the total operating torque.

The fact that thread friction is so high often suggests various devices that might decrease it, making large valves easier to operate. A widely used device in some non-power applications is the ball-bearing stem thread as used in automobile steering mechanisms. Unfortunately, valve stem lads turn out to be so high that the allowable contact pressure of any reasonable number of balls is exceeded. Also, the requirement for extreme hardness on both ball rolling surfaces is not generally compatible with the other valve construction features. A further deterrent is the fact that too low a friction factor at the valve stem thread could allow a valve to automatically fly open. Some locking device would be required if the ball-bearing thread valve is to be held tight in the closed position.

 

Frequency Study Required

Before final decisions are made, knowledge of functional characteristics of valve operators must be added to knowledge of closing forces. Probably frequency of operation is the first factor to consider before selecting an operator. Allowable investment is quite different for a valve that might be opened only once a year than for a valve that might be opened and closed once a day.

Urgency of operating the valve must be considered, too. Large valves are opened and closed only according to a pre-arranged schedule in many instances. As much as a week's warning might be available that the valve must be closed at a certain time. In other cases, valves are subject to emergency closure, demanded in a matter of a few seconds. This prohibits any setup in which the foreman must find a man and then send him off to a remote location to close the valve manually.

Still another consideration is the location of the valve. If it is easily accessible, there might be considerable difference between the cost of manual-operation and motor-operation. If the valve is in a very remote spot, manual-operation might call for platforms, scaffolds, stairways and railings that would decrease the cost differential between motor-operation and hand-operation. On the other hand, extension stems and chain wheels may achieve remote operation at relatively low cost.

A further consideration is whether the valve is always to be operated from a single station, or from several different stations at the same time.

About Motor Operators

Quite a number of different operating mechanisms are available to meet different requirements, ranging through a wide cost spread. Probably the most expensive, commonly used valve operator is the geared electric motor type, which closes the valve and cuts itself off when a predetermined torque has been developed by the motor. Such an operator is relatively expensive, both in equipment and electric wiring costs. Its advantages are tremendous, however.

For relatively little additional cost, it can be set up to be operated from several different locations. Open-and-close buttons can be installed in a control room, beside the valve, beside the boiler feed pump, plus any other desired location. Remote indicating devices can be attached at relatively low cost to show the position of the valve at any of the operating stations. By merely choosing a large enough motor, almost any operating speed can be achieved. There is no worry about power plant personnel becoming so tired that they cannot achieve complete closure of the valve within a specified time. The valve can be installed at almost any point in the power plant piping system, without regard to operating levels, access for service or proximity to other equipment. It is an ideal device for valves that are frequently operated.

 

Simple Handwheel Costs Least

At the other end of the cost scale is the simple handwheel attached directly to the operating nut or stem. This is certainly the lowest-cost operating device, having essentially none of the advantages of the motor drive. The obvious disadvantage arises from the high operating torque. In some of the typical examples already cited, the torque requirements on a simple 36-in. diameter wheel might call for 150 lbs pull on the rim of the wheel to turn the valve from open to close. As much as 1000 to 2000 lbs pull would be needed on the rim of the wheel to seat the disk. Thus, 20 men would have to crowd around the rim of this simple wheel to achieve the 2000-lb pull.

A common solution to this high-torque problem is to equip the valve with a gear system. Gears, of course, change the operating ratio down to a point where the pull required on the rim of the wheel is within the capability of one or two men. The number of revolutions required for given travel goes up corresponding to the change in gear ratio. In addition, friction losses in the gear train increase. So, the forces required may go down, but the total work effort goes up.

The peculiar force requirements of opening and closing a valve call for relatively low forces through a long distance, in going from open to close, followed by high forces through a short distance at the point of closure. This often suggests a gear shift device. Low gear would be used for developing the tight seating force and high gear for operating from open to close. The general idea is sound in theory. But, in practice, the cost of such a gear shift makes the idea impractical. Few devices of this type are available.

A mechanism with continuously variable gear ratio...giving high speed in the open position and low speed (with high force) in the near-closed position...is partially achieved by what is known as a toggle construction. A moderate number of large, high-pressure valves have been built, from time to time, with variations of toggle-type operating mechanisms. Such constructions are considerably preferred over a plain manually-operated wheel. But in large, high-pressure valves, toggle-type operators leave something to be desired. Total number of revolutions required from open to close tends to be relatively high, and the work load tends to be more than a single man can perform, even over periods as long as 15 or 20 minutes.

Still another form of gear shifting has been used on large, high-pressure valves for many years...the Impactor handwheel. It, in effect, is a directly-coupled handwheel on the valve operating nut, with a backup feature. This feature permits striking an impact blow. The valve is turned from open to close by continuously rotating the handwheel. No gearing or other magnification of the torque is applied. When the valve is approximately seated, the handwheel is backed off about 1/3 of a turn and then rapidly brought forward. Two anvils on the wheel strike a cross arm secured to the operating nut. The momentum of the spinning handwheel generates almost an infinite operating torque for an instant. By applying a series of such blows to a typical 36-in. Impactor handwheel, one man can develop torque equivalent to 5000 ft-lbs, with no gearing or other magnifying mechanism. This type of valve operator has proved very efficient in many years of high-pressure power plant use.

A recent development that bridges the gap between motorized operation and hand operation is the Impacto-gear operator. This is a modification of the widely used Impactor handwheel. A gear-and-pinion assembly is added to the valve operating construction. The pinion shaft projects from the side of the valve, below the handwheel. The pinion is turned by a portable motor, such as an air-driven wrench. A wrench on the end of one air hose could serve several valves. Gearing gives a 12 to 1 reduction.

A portable air wrench putting out 50 ft-lbs of torque will generate 600 ft-lbs of operating torque at the valve operating nut. This is sufficient to carry most large, high-pressure valves from full open to full close or back in one minute. Power plant operators are relieved of the backbreaking work of turning the valve handwheel through many revolutions with sustained effort that might range from 50 to 150 lbs pull on the rim of the wheel. After a valve equipped with an Impactogear operator has been brought near the seat, a series of impact handwheel blows are struck by hand to achieve the 3000 or 4000 ft-lb closing torque necessary for tight seating. Thus, one man can close a large, high-pressure valve in a reasonably short time without excessive manual effort. This does not replace the electric motor operator by any means. Though lower in cost, it does not offer remote operation or remote indication.

In the final analysis, the best choice of valve operator is dictated by recognizing torque requirements of the valve, the functional features of the many types of operators available, and the needs of the installation.

When considering relatively expensive electric motor operators, a very uneconomic selection might be made using highest possible test pressure as a criterion. In this case, valve purchaser and valve supplier must carefully study the exact application, and the maximum differential, that will be applied to the valve seat. Often, several thousand dollars can be saved by sizing the motor operator, exactly for the job application.