How to read flow characteristics
The best way to select a suitable pressure control valve for your application is through the valve's flow characteristics, which are provided by the manufacturers. In this context, flow characteristic is a misleading term, as pressure control valves do control pressure, not flow. The pressure control characteristic curve indicates the pressure range in which the pressure control valve operates at varying flow rates.
When selecting a pressure control valve, it is not only the dimensioning that is decisive. The desired function of the valve in the application is the sum of a wide number of properties. A pressure control characteristic curve visualizes the entirety of the valve's properties at a glance. Once you have understood a pressure control characteristic curve, it is quick and easy to read.
Unfortunately, a common way of selecting a control valve is to consider only the Kvs value of the valve. If, during the use of the valve in the system, the actual flow rate is within the range of the Kvs value, it appears that the pressure control valve is properly sized. However, this is not necessarily the case. The Kvs value of a valve simply describes the maximum possible flow through the valve. At maximum flow, however, the valve can no longer control the pressure. If volume flows are expected that could reach the Kvs value of the valve, the valve is not the suitable one for the intended application.
Basics
The main function of a pressure control valve is the constant control of a pressure on one side of the valve, while volume flow and pressure on the other side of the valve may vary considerably. A pressure reducing valve controls the outlet pressure of the valve, a back pressure regulator controls the inlet pressure of the valve. A back pressure regulator is no safety valve!
In this framework, the characteristics of a pressure reducing valve are considered by way of example, since they are usually used more frequently.
A pressure control characteristic curve illustrates the quality of the valve's pressure control as a function of the outlet pressure to be controlled depending on the volume flow. The volume flow is not controlled by the pressure control valve, but by a valve or similar downstream of the pressure control valve. The pressure control characteristic curve illustrates how the pressure control valve reacts to changing volume flows.
The above mentioned diagram shows pressure control characteristics determined by Mankenberg on the test bench for the DM 152 series. Most of the curves start at 2.5 bar(g). This corresponds to the set pressure of the valve. Although no adjustments are made to the set pressure, the characteristic curve shows a change in pressure as the volume flow fluctuates.
The pressure control valve automatically makes adjustments to the course of the characteristic curve in order to maintain the set pressure when the flow rate changes. However, no pressure control valve is perfect. If the flow rate increases, the pressure to be controlled downstream of the valve decreases slightly.
How to read pressure control characteristic curves
Determine the range of volume flow fluctuation you expect in your plant.
If necessary, you may need to standardise the flow rate to Kv values. You will find instructions for calculating the Kv value in our publication Design of Pressure Control Valves or calculate it with our design software ValvePilot under www.valvepilot.com. Mark the points in the diagram.
Select the characteristic curve with your desired set pressure and the pertaining inlet pressure.
Read the change in pressure along the characteristic curve within the range of volume flow variation you have specified. Is the pressure change acceptable to you? If it is not, you should select another pressure control valve.
Ideally, the pressure control valve operates in the flattest area of the pressure control characteristic curves, since even large changes in volume flow require only small pressure changes here. The outer ends of the pressure control characteristic curve are often characterized by rapid pressure drops. The pressure change in these areas is unusually high with small changes in volume flow. It is not recommended to operate the control valve in these areas.
Each pressure control valve has an almost unlimited number of characteristic curves. For this reason, be sure to consider the correct ones for your design. For any given set pressure there is a characteristic curve. The above mentioned diagram shows an example of a set of characteristic curves for the set pressure 2.5 bar(g) and a characteristic curve for the set pressure 1.7 bar(g) and 0.8 bar(g). This means that the setting range of the valve can also be read off from the characteristic curves. If you require a set pressure of the valve which lies within the setting range but for which no characteristic curve is available, you can interpolate. It is helpful that the characteristic curves themselves have a very similar shape and are only shifted on the y-axis.
As can be seen from the group of curves at the set point 2.5 bar(g), the shape of the pressure control characteristic curve is influenced not only by the set pressure but also by the inlet pressure. A pressure control characteristic curve can be determined for each permissible inlet pressure of the valve.
To select the correct curve for your system, first pay attention to the set pressure of the valve you have chosen, then to the correct inlet pressure and finally to the expected volume flow fluctuation. Interpolation of the curves for different set pressures and different inlet pressures is expressly permitted within the scope of the specified characteristic curves!
Finally, you should take a look at the units to make sure that the representation of the characteristic curves is shown in your standard units. The indication of the volumetric flow rate can differ greatly for gases and liquids. Where applicable, even mass flow rates are specified. In both cases, the specification of an operating medium is essential for valid pressure control characteristic curves. Special attention should also be paid to the specification of a medium-independent Kv value instead of a volume flow.
Features
The two outer ends of the pressure control characteristic curve shown at the side are characterized by steep pressure losses. Therefore, Mankenberg recommends aiming at a control range for the valve between 10 % and 70 % of the Kvs value.
The pressure loss at the beginning of the characteristic curve is called the closing pressure. The closing pressure is the additional pressure required to close the valve completely after its control activity.
The pressure loss at the end of the characteristic curve results from flow limitation, which occurs due to the maximum lift of the valve or phase transitions of the operating medium. The Kvs value is the highest measurable Kv value of the valve, which is usually at the extreme edge of the characteristic curve. In this range, the valve no longer controls. This is one reason why the design of a pressure control valve exclusively via the Kvs value is rarely useful.
For some valves it is important whether a pressure control characteristic curve is read from left to right (volume flow increase) or from right to left (volume flow decrease). The phenomenon that can occur here is called hysteresis and is shown in the second diagram. Both characteristic curves belong to the same set pressure and inlet pressure of the valve, only the reading direction changes.
Summary
When selecting a pressure control valve for your application, pay attention to the pressure control characteristic curves and not exclusively to the Kvs value. Ensure that you look at the pertaining characteristic curves for your application case.
- Is the correct set pressure displayed?
- Is the inlet pressure the right one?
- Are you working with the correct units?
- Do you have to make interpolations or a conversion of the volume flow, if necessary?
Once you have taken these aspects into account, you can enter the expected volume flow fluctuation in the diagram and determine the resulting pressure change. Ideally, the volume flow fluctuation is located in the flat area of the characteristic curve, since the pressure control valve consistently controls at a high level there. Avoid volume flows below 10 % of the Kvs value and above 70 % of the Kvs value.
If no pressure control characteristic curves are available for your selected valve or you need assistance with the design of your valves, please contact our experts who will advise you on the suitable valve design for your application.
