Valves selection

The valves used for chemical process plant can be divided into two broad classes, depending on their primary function  

Shut-off valves (block valves), whose purpose is to close off the flow. 

Control valves, both manual and automatic, used to regulate flow. 

A valve selected for shut-off purposes should give a positive seal in the closed position and minimum resistance to flow when open. Gate, plug and ball valves are most frequently used for this purpose. The selection of values is discussed by Merrick (1986) (1990), Smith and Vivian (1995) and Smith and Zappe (2003). 

If flow control is required, the valve should be capable of giving smooth control over the full range of flow, from fully open to closed. Globe valves are normally used, though the 

---

Implantable valves, particularly mechanical valves which continue to encroach on tissue valves, are unique. Methods such as valvuloplasty, mitral valve repair, or use of ultrasound are unlikely to reduce the number of valve replacements into the twenty-first century. Valve selection remains in the hands of the surgeon because of the critical nature of the procedure. If anything goes wrong, the result can be catastrophic to the patient. Cost of a valve, from 3000— 4000, is a relatively small part of the cost of open-heart surgery which can mn as high as 30,000. Growth of the cardiovascular valve market has slowed in the United States with the decline of the threat of rheumatic fever. 

Back pressure is included as a factor in PR valve selection and sizing accordingly ... 

In addition to valve efficiency, the following should be considered in valve selection ease of maintenance, durability, and spare parts required. 

Table 15-3 summarizes and compares the different valve types discussed in this chapter and highlights important properties that impact valve selection. 

Often the facility is designed with equipment and piping that can handle the complete range of operating conditions, and with control valves selected so that their internals ( trim ) can be substituted as operaiing... 

A Statistical Analysis of Nuclear Power Plant (Pump and Valve) Failure Rale Variability me Preliminary Results Nuclear All IPRDS data base records for the pumps and valves selected for analysis The set of valves and pumps selected for analysis from the IPRDS data base 104. 

The designer must examine the specific valve selected for a service and evaluate the materials of construction in contact with the process as well as in contact or exposed to the vent or discharge system. Sometimes the corrosive... 

If the pressure drop across the valve is to be more than 42 per cent of the inlet absolute pressure the valve selection is the same as if the pressure drop were only 42 per cent. With this pressure ratio the steam flow through the valve reaches a critical limit, with the steam flowing at sonic velocity, and lowering the downstream pressure below 58 per cent of the inlet absolute pressure gives no increase in flow rate. When the heater needs a higher pressure, or when the pressure required in the heater is not known, it is safer to allow a smaller pressure drop across the control valve. If the necessary heater pressure is not known, a pressure drop across the control valve of 10-25 per cent of the absolute inlet pressure usually ensures sufficient pressure within the heater. Of course, in the case of pressure-reducing valves the downstream pressure will be specified. 

Merrick, R. C. (1990) Valve Selection and Specification Guide (Spon.). 

SMITH, E. and Vivian, B. E. (1995) Valve Selection (Mechanical Engineering Publications). 

SMITH, P. and Zappe, R. W. (2003) Valve Selection Handbook, 5th edn (Gulf Publishing). 

Whenever a test 1s to be run, the sample composition and Instrument control parameters must be defined. This Is done with three (or more) data-entry screens. The first data-entry screen, shown In Figure 4, deals with experiment identification and base fluid composition. The operator simply types in the desired information Into unprotected fields of the screen. Information requested Includes such Items as experiment ID, submitter s name, base fluid type and base fluid additives. The base fluid pump rate and valve selection are also requested for later use by the control programs. The second data-entry screen is used to select the desired test temperatures and also to record any comments related to the experiment. The third data-entry screen Is used to input the in-line additive compositions. This screen is filled out for each set of additives to be tested with the base fluid as described on Data-Entry Screen No. 1. Also input are the pump rates for each of the three additive pumps. This information is used by the control programs when the additive set is being tested. (The pump rates are preset by the operator, but the pumps are turned on and off by the control programs as necessary during the course of an experiment.)... 

A reaction has been characterised as a tempered hybrid system and it has been determined that the system will relieve single-phase gas/, vapour. Relief sizes for both a safety valve and a bursting disc system are required. The reactor contains a charge of 3000 kg. Data for relief sizing have been compiled in Table A6.2. -The type of safety valve selected has a de-rated discharge coefficient under BS 6759131 of 0.87. 

Rangeability is the ratio of maximum to minimum flows over which the valve can give good control. This concept is difficult to quantify and is not used much for valve selection. A valve generally can be designed properly for a suitably wide flow range. 

R. W. Zappe, Valve Selection Handbook, Gulf Publishing Co., Houston, Tex., 1991. 

The relation between the valve stem position and the flow through the valve at constant pressure drop is termed the valve characteristic. Two characteristics must be evaluated for valve selection, the inherent and the installed characteristics 67. ... 

The point, however, is that an SRV should never be installed if there is a doubt about its suitability for the service. If the customer s HAZOP (hazard and operability) review shows some potential for blockage (caused by dirt, polymerization, hydrates formation, etc.), then the valve selected for installation should be such that it can still operate properly under these conditions. 

Again, it should be emphasized that SRVs must be selected by those who have complete knowledge of the pressure-relieving requirements of the system to be protected and the environmental conditions particular to the specific installation. Selection should not be based on arbitrarily assumed conditions or incomplete information. Valve selection and sizing (or sizing verification) is the responsibility of the system or process engineer and the user of the equipment to be protected. If unsure, seek professional guidance. 

After the value of R is determined (in Appendix G), the factor Kv is obtained from the graph in Appendix G. Factor Kv is applied to correct the preliminary required discharge area. If the corrected area exceeds the chosen effective orifice area, the above calculations should be repeated using the next larger effective orifice size, as the required effective orifice area of the valve selected cannot be less than the calculated required effective area. 

Chatter, however, is due to incorrect installation or valve selection and cannot be seen as an accidental occurrence. The noise can sometimes be worse than an opening valve, but this is impossible to calculate. Here, there are no official guidelines to avoid wrong valve selection or installation. Therefore, making a correct selection and avoiding wrong installation are also paramount in order to avoid noise from SRVs. 

---