Valve trim

Valve Trim Various alloys are available for valve parts such as seats, disks, and stems which must retain smooth finish For successful operation. The problem in seat materials is fivefold (1) resistance to corrosion by the fluid handled and to oxidation at high temperatures, (2) resistance to erosion by suspended solids in the fluid, (3) prevention of galling (seizure at point of contact) by differences in material or hardness or Both, (4) maintenance of high strength at high temperature, and (5) avoidance of distortion. 

When the layer of graphite and corrosion products is impervious to the solution, corrosion wdl cease or slow down. If the layer is porous, corrosion will progress by galvanic behavior between graphite and iron. The rate of this attack will be approximately that for the maximum penetration of steel by pitting. The layer of graphite formed may also be effective in reducing the g vanic action between cast iron and more noble alloys such as bronze used for valve trim and impellers in pumps. 

Figure 5-1 illustrates a method that will produce a system in which the parts fit together to accomplish the common goal of good control. Control valve share of total system flowing pressure drop will be 60% at normal flow. The system will still achieve maximum flow as long as the control valve trim selected can pass maximum flow at operating head loss (line 23 of Figure 5-1). The procedure described in Figure 5-1 is intended as a stand-alone device for guiding the calculations, and worksheets can be prepared from it.

Figure 10-13 illustrates the flow versus valve stem travel characteristic for various typical valve trim functions (Fisher Controls, 1987). The quick opening characteristic provides the maximum change in flow rate at low opening or stem travel, with a fairly linear relationship. As the valve approaches the wide open position, the change in flow with travel approaches zero. This is best suited for on-off control but is... 

Some general guidelines for the application of the proper valve characteristic are shown in Fig. 10-14. These are rules of thumb and the proper valve can be determined only by a complete analysis of the system in which the valve is to be used [see also Baumann (1991) for simplified guidelines]. We will illustrate how the valve trim characteristic interacts with the pump and system characteristics to affect the flow rate in the system and how to use this information to select the most appropriate valve trim. 

The range of flow rates possible with the control valve can be estimated by inserting the linear valve trim [i.e., Cv max/(T) = 64X] into Eq. (10-33) and calculating the system curves for the valve open, half open, and one-fourth open (X = 1, 0.5, 0.25). The intersection of these system curves with the pump curve shows that the operating range with this valve is approximately 150-450 gpm, as shown in Fig. 10-17. 

The basic reason for using different control-valve trims is to keep the stability of the control loop fairly constant over a wide range of flows. Linear-trim valves are used, for example, when the pressure drop over the control valve is fairly constant and a linear relationship exists between the controlled variable and the flow rate of the manipulated variable. Consider the flow of steam from a constant-pressure supply header. The steam flows into the shell side of a heat exchanger. A process liquid stream flows through the tube side and is heated by the steam. There is a linear relationship between the process outlet temperature and steam flow (with constant process flow rate and inlet temperature) since every pound of steam provides a certain amount of heat. 

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. 

Sand can be very troublesome in separators by cutting out valve trim, plugging separator internals, and accumulating in the bottom of vessels. Special hard trim can minimize effects of sand on valves. Accumulations can be alleviated by use of cone bottoms (for vertical vessels), sand jets and drains, as discussed earlier. 

The use of bursting disc devices on the downstream side of safety valves may be considered in cases where corrosion or fouling of the valve trim may be a concern (a common problem in systems using common headers to evacuate process media). 

It is important to mention that gas and vapours have different release characteristics than liquids, and that what is being discussed here is typical for gas, vapours and steam. Until 1985, the code allowed for liquid applications to have an overpressure of 25%. Until then, the same valve trim was used for both gases and liquids, resulting in the release characteristics shown in Figure 5.7. 

Opening and closing tolerances too short 1. Change valve trim and retest. 2. Use high performance valve. 

Although no known material will stand up to cavitation, some will last longer than others. The best overall selection for cavitation resistance is Stellite 6B (28% chromium, 4% tungsten, 1% carbon, 67% cobalt). This is a wrought material and can be welded to form valve trims in sizes up to 3 inches (75 mm). Stellite 6 is used for hard-facing of trims and has the same chemical composition but less impact resistance. Correspondingly, the cost of Stellite 6 is lower than the cost of Stellite 6B. 

Corrosion problems have also occurred in compressors, sour water valve trims, expansion bellows, and hydrocracker fractionators. Type 4140 (UNS G41400) compressor impellers have sulfide cracked in sour gas when the hardness exceeded 235 Brine)i(e>. Hardened 12Cr valve trim has sulfide cracked to prevent this problem, 18Cr-8Ni SS trim in sour water service is commonly used. Sulfide cracking has not occurred significantly in pumps. 

Hardness of completed carbon and low-alloy steel welds should not exceed 200 Brined.44 Valve trim shall be iBCr-gNi SS and meet MACE Standard MRQ17, m... 

Determine Cj for critical vapor flow. Assuming reduced valve trim for a heavy-duty angle valve, Cj = 0.55 from Table 16.2. 

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