Valve design sizing

The pressure drop across a steam control valve is a function of the valve design, size, and flow rate. The most accurate pressure-drop estimate that is usually available is that given in the valve manufacturer s engineering data for a specific valve size, type, and steam flow rate. Without such data, assume a pressure drop of 5 to 15 percent across the valve as a first approximation. This means that the pressure loss across this valve, assuming a 10 percent drop at the maximum steam flow rate, would be 0.10 x 80 = 8 psig (55.2 kPag). 

Although it has been common practice to specify the pressure loss in ordinary valves in terms of either equivalent length of straight pipe of the same size or velocity head loss, it is becoming more common to specify flow rate and pressure drop characteristics in the same terms as has been the practice for valves designed specifically for control service, namely, in terms of the valve coefficient, C. The flow coefficient of a valve is defined as the volume of Hquid at a specified density that flows through the fully opened valve with a unit pressure drop, eg, = 1 when 3.79 L/min (1 gal /min) pass through the valve... 

Valve Application Technology Functional requirements and the properties of the controlled fluid determine which valve and actuator types are best for a specific apphcation. If demands are modest and no unique valve features are required, the valve-design style selection may be determined solely by cost. If so, general-purpose globe or angle valves provide exceptional value, especially in sizes less than 3-inch NFS and hence are very popular. Beyond type selection, there are many other valve specifications that must be determined properly in order to ultimately yield-improved process control. 

Startup flow to the expander is designed to be supplied from the discharge of the compressor through a throttle valve and cooler. A careful study of the characteristics of the compressor blow-off valve and the expander startup valve was performed by Solvay process engineers. Based on these calculations, the startup cooler and valve were sized to supply the minimum required flow to tlie expander wheel. 

Whenever two-phase flow is encountered in facility piping it is usually in flowlines and interfield transfer lines. Some designers size liquid lines downstream of control valves as two-phase lines. The amount of gas involved in these lines is low and thus the lines are often sized as singlephase liquid lines. Oversizing two-phase lines can lead to increased slugging and thus as small a diameter as possible should be used consistent with pressure drop available and velocity constraints discussed in Volume 1. 

For conventional valves, pressure drop or variations in back pressure should not exceed 10% of set pressure. Because most process safety valves are sized for critical pressure conditions, the piping must accommodate the capacity required for valve relief and not have the pressure at the end of vent or manifold exceed the critical pressure. Designing for pressure 30% to 40% of critical w ith balanced valves, yields smaller pipes yet allows proper functioning of the valve. The discharge line size must not be smaller than the valve discharge. Check the manufacturer for valve performance under particular conditions, especially with balanced valves w hich can handle up to 70% to 80% of set pressure as back pressure. 

The design, size and siting of the boiler feed tank or hotwell must be compatible with the boiler duty capacity and system temperatures. They should be installed giving sufficient space for access to controls, valves and manways. 

Above 1000 Flow through valves may become critical factor Larger pump size selection run at reduced speed-e.g. 3 X size at 1000 cenhistokes running at one-third speed. Modification of valve design may be desirable for higher viscosities... 

There are four basic valve designs used in these compressors finger, channel, leaf, and annular ring. Within each class, there may be variations in design, depending upon operating speed and size of valve required. 

Typical manufacturer s values of Cv to be used with Eq. (10-29) require the variables to be expressed in the above units, with hv in ft. [For liquids, the value of 0.658 includes the value of the density of water, pw = 62.3 lbm/ft3, the ratio g/gc (which has a magnitude of 1), and 144 (in./ft)2]. For each valve design, tables for the values of the flow coefficients as a function of valve size and percent of valve opening are provided by the manufacturer (see Table 10-3, pages 318-319). In Table 10-3, Km applies to cavitating and flashing liquids and C applies to critical (choked) compressible flow, as discussed later. 

Where process, safety, and environmental considerations permit, vacuum protection may be provided by properly sized ever-open vents. Alternatively, active protective devices and systems are required. Vacuum breaker valves designed to open and admit air at a predetermined vacuum in the vessel are commonly used on storage tanks, but may not be suitable for some applications involving flammable liquids. Inert gas blanketing systems may be used if adequate capacity and reliability can be ensured. Where the source of the vacuum can be deenergized or isolated, suitably reliable safety instrumented systems (e.g, interlocks) can be provided. 

The control valve/pump sizing procedure proposed above is not without its limitations. The two design equations for the maximum and minimum conditions in general terms are ... 

The investigation revealed that the valve design did not recognize or compensate for the forces experienced in normal operation at 300 psig. Alternatively, the design did not specify the correct hardware for the service conditions. Use of nonstandard hardware should be avoided until such time that its operation is well understood and documented. There are certain hazards and risks associated with the use of specialty hardware and these are magnified when large size equipment is used. 

Because of dispersion through the water dump valve, oil size distribution at the outlet of a free water knockout or heater treater is not a significant design parameter. From dispersion theory, it can be shown that after passing through the dump valve, a maximum droplet diametet on the order of 10 to V) microns will exist, no matter what the droplet vi/e distribution was upstream of this valve. 

At an ASME-approved flow facility, a total of 9 valves of a particular valve design or range are flow tested at 10% overpressure above set pressure. They select three valve sizes and three set pressures. This way they establish the KD factor of each test valve, considering the flow conditions and each measured orifice area. 

If test certifications for spring-loaded valves on gas show the valve coefficient (API coefficient of discharge, Kd) to be less than 0.95, vendors proposals for that particular valve design shall be submitted to the end user for verification and approval of valve sizes. 

Most every flow control valve is sized 1 to 2 pipe sizes less than the pipe run the valve serves, because the valve restriction opening is considerably smaller than the connected pipe size. We size flow control on the basis of C (see Table 6.2). Most every valve vendor publishes C values for each valve they make. For good control tolerance choose a valve with a maximum C value at least 1.3 times greater than the calculated maximum flow C value. This means the designer should try 3 or more C values before setting a recommended size for the control valve. 

Representative Values of Relative Valve Capacity Coefficients (Cd) and of Other Sizing Factors for a Variety of Valve Designs. The Cd Values Listed Are for Valves with Full Area Trims, When the Valve Is Fully Open... 

In the dual-numbering system, one number refers to the valve design-type, and the other refers to its location and service. The first number, identifying the equipment, is unique to the SRV and is retired once the valve is taken out of service. Equipment records include information on the orifice size, set pressure, manufacturer, model number, serial number, and inlet and outlet flange sizes and ratings. 

TOWER SIZING GRAPHS AND CURVES IN NUTTER S FLOAT VALVE DESIGN HAVE BEEN FITTED Tp A SERIES OF EQUATIONS AND ARE USED IN THIS PROGRAM. 

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