Tight shutoff

Globe valves are used for gases, for control and wherever tight shutoff is required. Gate valves are for most other services. 

Standard valve trims are defined by standards such as API 600 and API 602. Elastomer or plastic inserts may be specified to achieve bubble-tight shutoff. Valve manufacturers may be consulted for recommended trims. 

When an SRV is subject to a minor pressure relief situation, the disc may only lift enough to cause a slighdy audible escape of fluid or visible drip (if liquid). This may relieve the system pressure, but the valve does not significantly pop or lift open. Under this condition, with metal seats the disc may not reseat properly and the valve may continue to leak below the system normal operating pressure. A resilient seat provides tight shutoff when the system pressure falls after a minor relief. 

Isolation A means for pressure-isolating control valves, pumps, and other piping hardware for installation and maintenance is another common application for an on/off valve. In this application, the valve is required to have tight shutoff so that leakage is stopped when the piping system is under repair. As the need to cycle the valve in this application is far less than that of a throttling control valve, the wear characteristics of the valve are less important. Also, because many are required in a plant, the isolation valve needs to be reliable, simple in design, and simple in operation. 

A particular type of booster relay, called a dead-band booster, is shown in Fig. 8-88. This booster is designed to be used exclusively between the output of a valve positioner and the input to a pneumatic actuator. It is designed to provide extra flow capacity to stroke the actuator faster than with the positioner alone. The dead-band booster is designed intentionally with a large dead band (approximately 5 percent of the input span), elastomer seats for tight shutoff, and an adjustable bypass valve connected between the input and output of the booster. The bypass valve is tuned to provide the best compromise between increased actuator stroking speed and positioner/actuator stability. 

In order to provide tight shutoff, extra force is needed, and therefore, the pressure difference on the diaphragm must rise. Consequently, at near-zero flow, the regulated pressure will rise. What the manufacturers call the "set point" of the regulator, in fact, is only the pressure at minimum flow (qf Maximum regulator capacity is not at full-valve opening (q2) but at maximum acceptable droop. Information on droop versus flow is therefore essential to check if regulator performance will be satisfactory. 

The simplest valve function is on-off service. Examples of this valve type are gate and ball valves, shown in Figure 8.2. A ball valve is used for tight shutoff. One application is a drain valve on a tank, where it is required to have the... 

Tight shutoff costs money. You can save on valve costs by understanding just what degree of shutoff is really needed. 

Chances are, that the plant gained nothing in performance, and that most of the valves are not even being maintained to tight shutoff levels. In short, thousands of dollars may have been wasted. 

Tight shutoff in a valve can cost a substantial amount, both initially and in maintenance. Considerable savings can often be realized by understanding the various ANSI leakage classes and by carefully determining the class truly needed in each application. 

A third factor is increased maintenance costs. There is considerable wear on the soft parts generally used to achieve tight shutoff. Therefore, to maintain the shutoff specification, it is necessary to rebuild the valve more often—leading to increased cost of maintenance personnel, repair parts,... 

Requiring tight shutoff in large valves is particularly expensive in applications where spring and diaphragm actuators are preferred or where positioners normally would not be required. This is due to the necessity of changing to piston actuators in order to get the necessary thrust. 

In a great majority of applications, including sliding-stem control valves, the implications of cost analysis are plain rather than arbitrarily specifying a tight-shutoff class, one should generally pick the lowest class required for the application. This will help ensure that the most cost-effective valve is chosen. 

This kind of valve is essential when tight shutoff is needed, particularly of gas flow. Multipass plug cocks, butterfly valves, slide valves, check valves, various quick-opening arrangements, etc. have limited and often indispensable applications, but will not be described here. 

---