The flow characteristics of a control valve show the rate of flow for the range of valve operation. There are two types of flow characteristics for control valves:
The inherent characteristic is determined by testing the valve flow versus valve lift using a constant differential pressure drop across the valve throughout the test. It is sure that after using the valve will not show the characteristics identical to that of manufacterer’s. In operation, the differential pressure across the valve varies throughout the valve position due to the system characteristics.
These curves in the picture below represent three basic characteristics
- Quick opening
- Equal percentage
These are the three curves which show the expected flow rate (Cv) for valve position. These are ideal or theoretical valve characteristics and generally change when the valve is installed. The differences in the characteristic curves can be due to the reproducibility of the valve. This includes the quality control of manufacturing tolerances.
The inherent characteristic is based on a constant differential pressure drop across the valve throughout the test. The installed characteristic differs in that the pressure drop changes in relation to the valve stroke.
When the valve opens and flow increases, the pressure drop across the valve decreases. Because of this, a valve which has an inherent equal percentage characteristic will produce a more linear flow curve when installed. This occurs as the reduced pressure drop counteracts the increasing flow area when the valve opens.
The installed characteristics are the important concern when considering valve performance.
The installed characteristics of a valve depends on how much of the total system pressure drop is across the valve. If all of the system pressure drop is across the valve, the installed characteristic is the same as the inherent characteristic. As the percentage pressure drop across the valve decreases, the installed characteristic of the linear valve approaches the quick opening response, and the installed characteristic of the equal percentage approaches that of the linear characteristic.
The installed valve characteristic can be modified to approximate the theoretical one using compensators in the control system.
if the control valve pressure drop is relatively constant then use a linear trim. However, if there is significant pressure variation as flow changes, then use equal-percentage trim
Equal Percentage is referred for control applications and also as increasing sensitivity as the flow rate increases at a greater rate for wider valve openings.
This characteristic also provides for high rangeability in a control valve. The advantage of a high rangeability control valve is that it increases the applications for use of the valve.
The linear characteristic is simply a proportional relationship between the flow rate and valve travel. Although rangeability is not as good as the equal percentage trim, they still provide high capacity. A linear characteristic may be a more suitable choice if the maximum flow capacity is the selecting factor.
This type of trim provides a majority of the flow as quickly as possible. Valves with this type of trim are normally used for ON-OFF control