PROCESS DISCIPLINE LUNCH & LEARN SESSION CONTROL VALVE SIZING (LIQUID SERVICE) PRESENTED BY WEST, E. F PROCESS DISCIPLINE LUNCH & LEARN SESSION CONTROL VALVE AND ITS OPERATION. • • Used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. Enable the direct control of flow rate and the significant control of process quantities such as pressure, temperature and liquid level. It is term as ‘’final control element’’. The opening and closing of automatic control valves is done by • Electrical- require cables and switch gear • Hydraulic - require pressure supply and return line for the hydraulic fluid. • Pneumatic - require compresses air supply. 2 PROCESS DISCIPLINE LUNCH & LEARN SESSION FEATURES OF CONTROL VALVE Control valve consists of three main parts. • Valve Actuator - moves the valve modulating element. • Valve Position - ensure that valve reached the desired end degree of opening • Valve Body – contains the modulating element. 3 PROCESS DISCIPLINE LUNCH & LEARN SESSION A TYPICAL CONTROL VALVE ARRANGEMENT 4 PROCESS DISCIPLINE LUNCH & LEARN SESSION TYPES OF CONTROL VALVES • • • • • • Globe Valve Angle seat Position Valve Butterfly Valve Ball Valve Pinch Valve Diaphragm Valve PROCESS DISCIPLINE LUNCH & LEARN SESSION LIQUID CONTROL VALVE SIZING TERMINOLOGY & EQUATIONS Flow Capacity The valve sizing coefficient most commonly used as a measure of the capacity of the body and trim of a control valve is the Cv. Control valves are sized using a special form of the orifice equation which gives the valve orifice size as a “valve flow coefficient” or Cv. The Cv is defined as the flow rate of water in US gallons per minute that can pass through a valve with a pressure drop of 1 psi at a temperature of 60°F. When selecting a control valve for an application, the calculated Cv is used to determine the valve size and the trim size that will allow the valve to pass the desired flow rate and provide stable control of the process fluid. The basic liquid sizing equation for calculating the Cv in US units is as shown in the next slide. PROCESS DISCIPLINE LUNCH & LEARN SESSION where CV = valve flow coefficient Q = volumetric flow rate (US gpm) FP = piping geometric factor SG = specific gravity of liquid relative to water ΔPeff = effective pressure drop across valve (psi) PROCESS DISCIPLINE LUNCH & LEARN SESSION Effective Pressure Drop The effective pressure drop across a liquid control valve depends on the nature of the liquid flowing through the valve and the valve design. If the pressures upstream, inside and downstream of the control valve are greater than the vapour pressure of the liquid at the flowing temperature, the effective pressure drop is equal to the actual pressure difference between the upstream and downstream sides of the valve. In this case, the flow is said to be “subcritical”. However, if the liquid vapour pressure exceeds the system pressure inside or downstream of the valve, vapourisation will occur and the flow will become “critical”. In this case, the effective pressure drop across the valve will be limited by the valve design and the physical properties of the liquid. PROCESS DISCIPLINE LUNCH & LEARN SESSION Flow is sub-critical if: PROCESS DISCIPLINE LUNCH & LEARN SESSION For critical flow: ΔPeff = ΔPmax Valve Liquid Pressure Recovery Factor, FL The valve liquid pressure recovery factor is the ratio of effective pressure drop to the pressure difference between the upstream pressure and the vena contracta pressure. The valve liquid pressure recovery factor is usually measured experimentally and is tabulated in valve manufacturers’ catalogues. Liquid Critical Pressure Ratio Factor, FF The liquid critical pressure ratio factor is a means of estimating the pressure at the vena contracta of the valve under critical flow conditions. PROCESS DISCIPLINE LUNCH & LEARN SESSION PROCESS DISCIPLINE LUNCH & LEARN SESSION Reynolds Number Factor, FR Reynolds number is a correction factor to account for non turbulent flowing conditions within the control valve to be sized. Such conditions might occur due to high viscosity fluid, very low pressure differential, low flow or some combination of these. PROCESS DISCIPLINE LUNCH & LEARN SESSION LIQUID CONTROL VALVE SIZING EQUATIONS PROCESS DISCIPLINE LUNCH & LEARN SESSION STEPS TO SIZE CONTROL VALVE (LIQUID SERVICE) • • • • • • • • • Specify the required design flow rate Specify the allowable pressure drop across the valve Choose a valve type and body size from the manufacturers’ tables Calculate the first estimate of the piping geometry factor Determine if the flow through the valve will be sub-critical or critical. That is, will some of the liquid vapourise causing flashing or cavitation? Calculate the effective pressure drop across the valve Calculate the first estimate of the required valve Cv Check that the calculated Cv is less than the actual Cv of the selected valve (re-select suitable valve from manufacturers’ tables if required) If the Cv and control range are suitable the valve is correctly sized. If not reselect another valve and repeat the sizing procedure from Step 3 PROCESS DISCIPLINE LUNCH & LEARN SESSION CASE PROBLEM SIZING INPUT DATA To size a control valve for the following application below: Fluid: Liquid Propane Design flow rate, Q = 800 US gpm Upstream pressure, P1 = 314.7 psia (21.7 bar) Downstream pressure, P2 = 289.7 psia (19.97 bar) Liquid temperature = 70°F (21.11 °C) Propane specific gravity, SG = 0.5 Propane vapour pressure, PV = 124.3 psia (8.57 bar) Propane critical pressure, PC = 616.3 psia (42.49 bar) Pipe size = 8 inch Desired valve design—Class 300 Globe valve and an assumed valve size of 2 inches. • • • • Calculation Following the steps given in one of the slide: Design flow rate = 800 US gpm Effective pressure drop across valve, ΔPeff= P1 –P2 = 25 psi (1.73 bar) CL 300 2” globe valve as the preliminary selection. Valve size coefficient at 80% travel for the assumed valve size, CV = 69.8 PROCESS DISCIPLINE LUNCH & LEARN SESSION Use of spreadsheet PROCESS DISCIPLINE LUNCH & LEARN SESSION Valve Coefficient CV Table PROCESS DISCIPLINE LUNCH & LEARN SESSION Equation Constants Table PROCESS DISCIPLINE LUNCH & LEARN SESSION Thank You
