Control valve pressure drop Calculations

TTie loss coefficient (K) of fittings and block valves are function of Reynolds number and the nominal line size (dfl, in inch). The three K (Kl, Ki, and Kd> and equivalent line length (U,) of different fittings and block valves are listed in Table 3. Eq. (30b) is used to calculate control valve K with known inlet line tnside diameter D (in fl), and control valve sizing coefficient (Cv). Control valve pressure drop calculation by this method is a rough estimate, For more accurate control valve pressure drop calculation, the methods in Chapter 7 should be used. 

Pressure drop, chart, 102, 103, 111 Computer aided drafting, 17 Condensate, flashing flow, 135-142, 147 Charts, 142, 143 Control valve pressure drop, 90 Calculations, 90-96 Cost estimates, plant, 45-49 Accounting, 48... 

Please first note that these calculated answers have been rounded off conservatively to the higher values. The reason is that greater acuracy is not justified. The control valve pressure drop selected should be within a tolerance of 10% of the total system pressure loss the control valve governs. 

The computer program PROG52 calculates the control valve capacity coefficient (C ) or the maximum flow rate that will pass through the valve. Table 5-9 shows the input data and results of the control valve calculations for liquid, gas, and steam conditions. For the liquid service, at a flow rate of 400 gpm and the control valve pressure drop of 25 psi, the control valve capacity coefficient (CJ is 80. The gas service with a flow rate of 1200 fP/h, and control valve pressure drop of 400 psi gives the control valve capacity coefficient (C ) of 0.059. For the steam condition of capacity coefficient (C, ) = 75, and the control valve pressure drop of 10 psi, the maximum steam flow rate through the valve is 5511.4 Ib/hr. 

For a given liquid flow rate (Q), in gpm, and control valve pressure drop (dp), in psi, control valve sizing eoefScient (Cv) can be calculated by following equation ... 

Flow through each control valve usually has three difierent capacities maximum, normal, and miniinuin. Hydraulic study is usually either based on maximum flow or normal How. From hydraulic study (without pump or compressor available pressure drop of the control valve at cither condition (case 0) is calculated. The available control valve pressure drop at other two flow conditions (case 1 or 2) can be estimated by following equation ... 

Once control valve pressure drop at maximum, normal, and minimum flow cases are known. Required valve sizing coefTicient at each flow cases can be calculated. 

In preparation for exporting the steady-state flowsheet from Aspen Plus into Aspen Dynamics, all equipment is sized. Column diameters are calculated by Aspen Tray Sizing. Pumps heads and control valve pressure drops are specified to give adequate dynamic rangeability. Typical valve pressure drops are 2 atm. The prereactor heat removal is specified to use a constant coolant temperature. 

Despite the need for good control in many process systems, most engineers do not allow the proper pressure drop for the control valves into their calculations. Many literature sources ignore the problem, and many plant operators and engineers wonder why the actual plant has control problems. 

Most line has pipe fittings, such as elbows, tees, reducers. It also has block valves, check valves, or instruments, such as control valve and oriflce flow meter. They will cause pressure drop in tine. In this section, we will discuss how to calculate pressure drop thru pipe fltting, block valve, check valve, and control valve using loss coefficient, K, since their pressure drops are required in hydraulic loop pressure profile calculation. Darby s 3-K. method will be used to calculate these pressure drops [ 12]. It is better than two K method or equivalent line length methods. Fittings and valves pressure drop (in terms of pressure head, H) is calculated by flillowing equation, ... 

Care must be taken to use the control valve CV to calculate the pressure drop. For fixed CV, with an increase in flow rate, the pressure drop increases (pressure drop increases with the square of the volumetric flow rate). In most cases, the control valve is used for a fixed pressure drop (in fact, with increase in flow rate, pressure drop through the control valve decreases). The designer must consider this fact to estimate the pressure drop through the control... 

Flow Summary shows the values of fC-pipe and fC-fittings. These are the indications of pressure drop contribution by the pipe and fittings. In the Pressure Drop frame, there are a few pressure drop calculations, which include the contribution of static pressure drop (elevation loss or gain), velocity head (change in velocity due to reducer or expander), and control valve. The result also shows the frictional pressure drop per 100 m. This is sometimes very useful, because most operating companies establish some basic value of pressure drop per 100 m. 

Choking, or expansion of gas from a high pressure to a lower pressure, is generally required for control of gas flow rates. Choking is achieved by the use of a choke or a control valve. The pressure drop causes a decrease in the gas temperature, thus hydrates can form at the choke or control valve. The best way to calculate the temperature drop is to use a simulation computer program. The program will perform a flash calculation, internally balancing enthalpy. It will calculate the temperature downstream of the choke, which assures that the enthalpy of the mixture of gas and liquid upstream of the choke equals the enthalpy of the new mixture of more gas and less liquid downstream of the choke. 

If calculated pressure drop from inlet to outlet of line system (not including control or hand valves) is greater than approximately 10%, but less than about 40% of the inlet pre.s.sure Pi (pounds per. square inch gauge), the Darcy equation will yield reasonable accuracy when using a specific volume based on the average of upstream (inlet) and do vnstream (outlet)... 

If pressure drop is too large, re-estimate line size and repeat calculations (.see paragraph (A) above) and also examine pressure drop assumptions for orifices and control valves. 

The choice of a suitable temperature or pressure control valve for steam application will depend on the supply side pressure, the downstream pressure, and the flow rate of steam to be passed. In the case of temperature control valves the first of these is usually known and the third can be calculated, but the appropriate pressure drop through the valve is often to be decided. Sometimes the maker s rating of a heater will specify that it transfers heat at a certain rate when supplied with steam at a certain pressure. This pressure is then the pressure downstream of the control valve, and the valve may be selected on this basis. 

The pressure (PSI) for calculating heat generation in a flow control valve, for example, is the inlet minus the outlet pressure, or the pressure drop across the valve. 

If the maximum flow-rate required is 20,000 kg/h, calculate the pump motor rating (power) needed. Take the pump efficiency as 70 per cent and allow for a pressure drop of 0.5 bar across the control valve and a loss of 10 velocity heads across the orifice. 

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.

All three types of valves described above are on-off type devices (binary devices). A basic requirement in the design of these valves is that they offer minimum resistance to flow when open. For many types of calculations it is often justifiable to neglect pressure losses through such devices. But such a simplification cannot be applied to flow control valves. With these devices the control of flow is accomplished either by a constriction or by a diversion. In either case an additional resistance to the flow is introduced. Globe, angle, cross, and needle valves are typical devices of this type. Specific pressure drop correlations should be developed for such devices and used whenever possible. One such correlation used by Stoner (S5) takes the form... 

Calculate the C value of the control valve, the pump head at design rate, the size of the motor required to drive the pump, the fraction that the valve is open at design, and the pressure drop over the valve at design rate. 

Here is a problem that came up on a sulfur recovery facility in Punto Fijo, Venezuela. The combustion air blower, shown in Fig. 28.8, was a fixed-speed, motor-driven centrifugal machine. The air intake filters were severely fouled. They had a pressure drop of about 8 in H/). The atmospheric vent valve, used to control the discharge pressure at a constant 12 psig, was 50 percent open. The unit engineer had been asked to calculate the incentive in electrical power savings that would result from cleaning the filters. 

The next step is to calculate the pressure loss to the control valve entrance. The designer has located the flow control valve as close as practical to the stabilizer feed entrance. This is a good design location for the control valve. Why The pressure drop across the control valve results in a two-phase flow, vapor and liquid flowing into the stabilizer feed tray. Thus, the downstream flow is a two-phase flow, and this problem will be finished later, in the section on two-phase flow. 

The pressure drop in a process is the summation of the pressure drop in the lines, that contributed by two control valves, the pressure drop in the column, and the pressure drop in the condenser. What is the precision of the calculated total pressure drop in psia calculated at a 95 percent confidence level for the following data ... 

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