9.4 Valve pressure drop
Valve pressure drop
Valve pressure drop constitutes a considerable proportion of the piping system head losses in pump installations which use valves as a means of controlling the flow. The system curve which is the basis for the determination of the required delivery head for the pump, cannot be defined with sufficient accuracy if the pressure drop across the valves, particularly control valves, is not considered. The following considerations apply to the choice of valves and the pressure drop across the valves for the purpose of determining the required pump performance. A more detailed treatment of flow is to be found in Chapter 10, Flow regulation.
Stop valves and check (non-return) valves
The term “stop valves” applies to those valves which are intended to be either fully open or fully closed. Stop valves are installed in an installation in order to make components accessible for service; to direct flow in another direction and for other similar applications.
The decisive requirements of a stop valve are low pressure drop in the open position and good leak-free sealing in the shut position. Information regarding the calculation of pressure drop in the fully open position is given in Chapter 8, Liquid flow >>>. The most reliable method, however, is to obtain the value of the loss coefficient from the valve manufacturer.
Check, non-return, valves are installed in a pipe system in order to prevent the back-flow of liquids. There are many different types of check valves available. Of the possible choices of check valves for a particular application, it is normal to select the one which gives least pressure drop. In some cases it is necessary to have exceptionally fast closing check valves. These are biased (by means of spring loading for example) and give rise to large pressure drops. Since the effective through-flow area varies with the flow through the valves, the loss coefficient also varies with flow in the case of check valves. Pressure drop calculations are best made by using data supplied by the manufacturer of the valve in question.
Control valves
The purpose of control valves, in contrast to the other valve groups, is to create head losses and thereby regulate the flow in the pipework system. The choice of valve is therefore made on the basis of entirely different criteria than for stop or non-return valves.
The principle requirements of a control valve is based upon its regulating characteristics. These characteristics are also dependent upon the interaction between the pump, control valves, pipe system and other control equipment. The valve can only regulate flow by “slowing it down”, i.e. by creating a greater or smaller pressure drop in the system. The capability of the valve to regulate ceases when the pressure drop across the valve approaches the pressure drop for the fully open position. If it is required to maintain in regulation capacity even at Qmax (maximum flow in the proposed process), the valve must therefore have an available pressure drop at that flow which is somewhat greater than that for the fully open valve position.
The design of other control equipment is simplified if the flow through the pipe system has fairly linear characteristics in relation to the position of the valve. This requirement also makes a large pressure drop across the valve desirable at Qmax.
The flow through a valve is calculated using the formula:
where
Q = volume flow (m³/s)
AV = valve flow coefficient at given position ( % of stroke)
p = density of the liquid (kg/m³)
ΔP = pressure drop across the valve (Pa)
A measure of the valve capacity and size is its flow coefficient at fully open position (100% stroke). In order to make a rapid assessment of a suitable valve size the following rule of thumb is used:
where
AV100% = flow coefficient at fully open position, 100% stroke
Qmax = maximum flow with regulation requirement (m³/s)
g = acceleration due to gravity 9,806 (m/s²)
Hstat = system static head (m)
hL pipe = total head loss for pipe system at Qmax excluding valve head loss (m)
If the two sides of the equation are equal then this indicates that the valve at full stroke would just precisely allow the flow Qmax to pass through and constitutes a theoretical lower limit of valve capacity. The given pressure drop at Qmax is a compromise between, achieving good regulating characteristics for the system without using too large a valve on the one hand and not necessitating too high additional pump delivery head with associated pump and energy costs on the other hand.
The next step is to select the valve characteristic. The two most usual are linear and logarithmic (equal percentage). In principle a valve can be given any characteristic by specially shaping the internal components (the valve trim i.e. plug etc.). The valve is described as having a linear characteristic if its flow coefficient increases in direct proportion to the stroke. If AV increases logarithmic in relation to the stroke then the valve is referred to as having an equal percentage characteristic.
Between these two types are valves with quadratic (V-port) characteristics. Many valves display characteristics which differ to a greater or lesser extent from these mathematically defined forms.
The following rule of thumb is used as a guide when selecting a suitable valve characteristic:
If the relationship between the valve pressure drop at Qmin (minimum flow which is to be regulated) and at Qmax is less than 3, try a valve having a linear characteristic. If ΔPQmin / ΔPQmax > 3 choose a logarithmic characteristic.
With a lower limit of AV100% and with a desired valve characteristic, a preliminary choice of control valve can be made.
Alternatives to AV are used in practice:
In catalogues, the given value for AV, KV and CV are derived by testing with water at 5-30°C, which must be considered when selecting a valve for high viscosity or non-Newtonian liquids.
Having made a preliminary selection of pump and control valve and after calculating the system curve excluding the valve, it is possible to check the pump installation regulating characteristics.
Example:
For a particular pump the system curve, excluding the valve, is determined (figure 9.4b upper illustration). The flow is to be valve regulated within the range 0,03-0,1 m³/s (108-360 m³/h). Select valve and pump size and check the regulating characteristics of the pump installation.
At maximum flow with regulating requirements:
Qmax = 0,1 m³/s, Hstat = 18m, hL pipe = 20m
According to the rule of thumb (equ. 9.4b) at Qmax
hV = 0,1 * 18 + 0,3 * 20 = 7.8m
where, hV = Valve head loss (valve pressure drop) in meters (m) at Qmax
Select a pump whose Q-H curve passes through the point, H = 18+20+7.8 = 45.8, at Qmax= 0,1 m³/s.
According to the rule of thumb (equ.9.4b)
From the pump and system curves the largest and smallest pressure drop across the valves within the regulating range can be read. The relationship between these is 35 / 7.8 = 4.5 which is > 3 and indicates a logarithmic valve characteristic.
Choose a regulating valve with AV100% = 0,013 having a logarithmic (equal percentage) characteristic. Calculate the valve setting (% stroke) at various flows by using the formula:
where
s = actual setting (stroke) in %
AVO = flow coefficient at s=0 (m³)
The calculated result is shown by the lower illustration in figure 9.4b. In the example AVO = 0,04 * AV100% ,i.e. the valve regulating range is 1:25. For comparison the pump installation regulating characteristics have also been calculated for a valve having linear characteristics. For the linear valve:
The largest flow, which can pass through the pump installation, is 0,102 m³/s and corresponds to the fully open valve position. Within the regulating range the valve stroke varies between 35 and 96 percent of full stroke for the equal percentage valve. Corresponding figures for the linear valve are 9 and 87 percent. The equal percentage valve gives the installation an approximately linear characteristic.
The chain-dotted curves in the lower illustration in figure 9.4b show the installation’s regulating characteristics at constant pressure drop across the valve, i.e. without considering the effects of the pump and the shape of the system curve.
Note that the pump data selected in the example neglects tolerances of the installation components, which is not to be recommended in reality.
Figure 9.4b Checking the regulating characteristics of the pump installation.
Summary, valve pressure drop
- The pressure drop across control valves is considerable and cannot be neglected when determining pump data.
- The regulating characteristics of the installation determines the pressure drop required across a control valve.
- The smaller the valve pressure drop in relation to the pipe system losses, the more distorted the valve characteristics.
- Valve pressure drop must be paid for with increased pump power and can cause considerable energy costs.
- Rules of thumb can never be applied universally.
- All components in the pump installation are subject to certain hydraulic tolerances. Deviations from the nominal performance for pumps, control valves and other fittings, together with inaccuracies when calculating pipe losses etc. are unavoidable and must be given careful consideration. In valve regulated systems the control valve is the component which is used to compensate for the component tolerances.