Almost every process cooling application that uses water or water/glycol will require some form of flow control. This article will explore automated on-off valves — sometimes called block valves — with a special emphasis on the angle-body valve. The most common block valves in the 0.5 to 3” NPS range are ball valves, butterfly valves and angle-body valves.
Many factors must be considered when selecting a valve, but the primary consideration is the process conditions that the valve will encounter. Secondary factors include the energy efficiency of the valve (because any valve requires a certain amount of energy to open or close), maintenance, and the effect of valve closure and water hammer.
Process Conditions Influence Valve Selection
When considering the process conditions, the flow rate of the process fluid through the valve is the first concern. The higher the flow rate through the valve, the higher the pressure loss will be. Typically, a block valve should not contribute to more than 1 percent of the pressure generated by the process pump. An angle-seat valve has a pressure drop similar to a reduced-port ball valve under similar flow conditions. The angle-seat valve will have a slightly higher pressure drop than a full-port ball valve or a butterfly valve.
The temperature and pressure that a valve will encounter during normal and upset conditions must next be considered. What also must be considered are rapid changes in pressure and temperature such as the introduction of steam into a cold valve or the introduction of a cold liquefied gas into a valve at ambient temperature.
The fluid physical properties are the next factor that should be considered in valve selection. Is the fluid water, water/glycol, heat transfer oil or refrigerant? What are the properties of the fluid (density, viscosity, corrosiveness), and do these properties change with a change in the temperature? The fluid density and fluid viscosity, along with the flow rate, will determine the pressure drop through the valve. The corrosiveness of the fluid will influence the valve’s materials of construction.
Valve Body Materials of Construction
A high chromium stainless steel is an excellent choice for a valve body. Although it is slightly more costly than the alternatives of bronze and steel, stainless is stronger and, as a result, less material can be used to achieve the same pressure and temperature rating. High chromium stainless steel does not elongate like bronze does under temperature increases. Stainless steel’s resistance to corrosion is well known, and it is resistant to the effects of rapid temperature and pressure changes. Stainless steel angle-seat valves typically are less costly than stainless steel body ball valves.
Cooling tower water usually contains a large amount of solids. These solids will quickly degrade close-tolerance valves such as ball valves. Angle-seat valves are not greatly affected by solids.
Valve maintenance is a major concern. Angle-seat valves are known for their durability and long service life. The two most common failure points in a valve are the sealing seat and product leakage from the valve stem.
The soft material used to seal the control element shaft is called the packing. It prevents the process fluid from escaping around the closure element shaft stem. The packing is compressed by the packing follower. Under pressure, the solid packing will exhibit characteristics of a fluid. The linear pressure from the packing follower will be converted into axial movement and pressure. This is the mechanism that seals the stem. However, as the packing wears, additional force must be applied to the packing to maintain the constant axial force. This is accomplished by tightening the packing nut or turning down the bolts on the gland flange.
Some ball valves and butterfly valves will employ a small degree of live loading, or constant tension, by means of a Bellville washer. However, this mechanism only provides tension for a period of time before the movement of the washer is exhausted.
The angle-body valve also requires a stem seal. However, in place of a Bellville washer is a long coil spring. The long bonnet bore allows for additional packing rings, and the coil spring can maintain constant compression regardless of the packing wear. A life of over 5 million cycles is guaranteed by one angle-seat manufacturer. The centrally located spring also provides compression of the packing, which seals the shaft entering the actuator.
Role of the Actuator with Process Valves
Typically, ball valves, butterfly valves and angle-body valves will use an air actuator to open and close the valve. An angle-body valve with a pneumatic actuator consumes less energy in the form of compressed air than does a similarly sized butterfly valve and consumes far less energy than a ball valve.
A butterfly valve is a quarter-turn device. The air actuator on a butterfly valve must convert linear motion to rotary motion. The typical rack-and-pinion mechanism in the air actuator has a low energy conversion coefficient. The butterfly valve actuator has a stroke that is longer than the stroke of an angle-seat valve. Air consumption is a function of the stroke length and the actuator cylinder cross-sectional area. The butterfly valve actuator will consume more air than an angle-body valve actuator.
The power requirement for opening and closing a ball valve is much greater than either a butterfly or an angle-seat valve. The ball is pressed against the seat material on the downstream side of the valve. There is sliding friction between the ball and the seat while the ball turns into position. There is even a greater amount of friction to overcome just to start the ball turning. This is called the breakaway torque.
The actuator cylinder diameter of a ball valve is far greater than the cylinder diameter of a butterfly or angle-seat valve. As a general rule, the larger the diameter, the larger the air consumption.
A unique feature of an angle-seat valve is its ability to prevent water hammer. The closing action of an angle-seat valve prevents damage caused by water hammer. Liquid flow in an angle-seat valve is directed to the underside of the valve seat. The buildup of pressure at the valve seat will prevent the rapid closure. The excess pressure is vented through the valve seat so water hammer cannot occur.
Angle-seat valves are available in a variety of end connections, NPT, weld, etc. Angle-seat valves have soft seats that allow them to provide bubble-tight Class VI shut off. Their linear actuator allows for the easy integration of valve control heads. There is even a variation of the angle-seat valve with a globe-style control element that provides the ability to perform 0 to 100 percent flow control. The angle-seat valve should always be considered whenever making a cooling water valve selection.