Control valve hydraulics is a key step in specifying a control valve in Process industry. A proper and comprehensive hydraulics involves not only determining the optimum pressure drop across the control valve for maximum, minimum and normal flow rates, but also checking for the possibility of cavitation in the valve and deciding the type of the valve. It can also help in selecting the flow characteristics type of the valve. If the pressure drop is too high, it means the loss of energy. On the other hand, if pressure drop is too low, it could compromise the controllability of the control valve. Hence, an optimum pressure drop should be determined.

A control valve’s controllability can be considered acceptable if it’s opening at maximum flow rate does not exceed 80-90% of full opening, and the opening at minimum flow does not fall below 10-20% of the full opening. If the pressure drop is too low, the opening at maximum flow may exceed the said range and at minimum flow might fall below the said range leading to poor controllability of the control valve.

Thus, the optimum pressure drop across a control valve involves a trade-off between the cost of energy and the controllability of the valve. This optimum pressure drop is a certain percentage of the total pressure drop across the control valve circuit (including control valves, pipes, fittings, any equipments and instruments) depending upon the range between the maximum flow and minimum flow which control valve has to handle.

In most of the common cases, a control valve handles maximum flow and minimum flow at 110% and 50% of the normal flow as plant turn-down is generally 50% and 10% margin over normal flow is the common consideration. In such cases, a control valve pressure drop of 25-30% of total dynamic losses across the control valve circuit will produce reasonably good controllability of the valve. This pressure drop would increase or decrease if the range between maximum flow and normal flow increases or decreases.

The controllability of a control valve can be evaluated using flow coefficient of the control valve. Flow coefficient (Cv) is defined as volumetric flow rate in gallons per minute of water through the valve at 60 deg F when pressure drop across the valve is one psi. Cv can be calculated using formula given in the standard ISA-75.01.01-2007.

To achieve the desired controllability, the ratio of maximum flow coefficient (Cv) to minimum Cv should preferably be not more than 15. If this ratio far exceeds the said value, the travel at minimum flow may be below 10% of the rated travel or the travel at maximum flow may exceed 90% of the rated travel, which means poor controllability. In that event, pressure drop across the control valve should be increased so that the said ratio can be lowered.

A comprehensive and thorough hydraulics should also involve preliminary size check of the control valve. Though the size of the control valve is determined by the control valve vendor, it’s worthwhile to do the preliminary check of the valve size during design phase itself to avoid any possibility of size becoming abnormal i.e. either higher than the pipe size or too less than the pipe size during the later phase of the engineering.

This can be done by estimating the rated Cv of the valve. For this Cv, given type of the valve and given pressure rating, control valve size can be obtained by referring to a control valve manufacturer catalogue. If this size is higher than the pipe size, the pressure drop across the control valve should be increased. On the other hand, if the estimated size of the control valve is too less than the pipe size, the pressure drop across the control valve should be lowered without compromising its controllability. This step may also help the selection of the right type of the valve. For example, changing the globe valve to butterfly valve or vice versa could yield proper size as well as controllability of the valve.

Control valve hydraulics should also check for the possibility of cavitation in the valve. During the flow through a control valve, the minimum pressure occurs at the vena contracta and then pressure increases along the path of flow till the outlet of the control valve. Vena contracta is a point in the flow path where flow area is the minimum. Therefore, velocity is the maximum and, hence, pressure is the minimum at vena contracta. For a liquid flow, if the pressure at the vena contracta is less than vapor pressure of the liquid, vapor bubbles are formed. As pressure recovery takes place downstream the vena contracta, the vapor condenses and the bubbles collapse. As bubbles collapse, it causes impact on the valve body and may cause erosion, noise and vibration. This phenomenon is called cavitation.

Full cavitation occurs when pressure drop across the control valve is more than or equal to certain minimum pressure drop (or critical pressure drop) and the pressure at the outlet of the control valve is more than the vapor pressure of the liquid.

A Process engineer should try to avoid the possibility of cavitation while specifying the control valve. Possibility of the cavitation can sometimes be avoided by making certain adjustments during hydraulics. For example, pressure drop across the valve can be fixed in such a way that it is less than the critical pressure drop, off-course without compromising the controllability of the valve.

Control valve hydraulics can sometimes also help in deciding the flow characteristics of the valve. Most common types of inherent flow characteristics are linear, equal percentage and quick opening. Pressure drop is one of the factors that help determine the flow characteristics type. Linear characteristic should be specified if most of the pressure drop as proportion of total pressure drop in the system is across the valve itself so that pressure drop across the valve remains nearly constant for varying flow rates. Equal percentage characteristic should be specified where high proportion of the pressure drop is in the system other than the valve i.e. in pipes, fittings, equipments etc.

Date of Publication: 16 September, 2017

About Author: Satyendra Kumar Singh, B.Tech. (Chemical Technology) + M.B.A., is proprietor of Satsha Management Services-an award winning design engineering and management consulting company (www.satshamanagement.com). He possesses approximately 30 years’ experience in engineering consultancy in process and energy industries. Satyendra has authored several papers on energy, business and management, which have been published in some renowned journals/magazines such as ‘Chemical Engineering’, ‘Process Worldwide’, ‘Modern Manufacturing India’. He may be reached at satyendra.singh@satshamanagement.com, Ph. +919811293605.