[From：http://instrumentation.web.id] [Author：admin] [Date：11-07-11] [Hit：]
Droop or offset at pressure regulator valves is defined by Fisher regulator as the amount a regulator deviates below its setpoint as flow increases. It’s an inherent characteristic of pressure regulator valves. For example, we have a pressure regulator valve set at 100 psig, but when this pressure regulator valve operate at it’s maximum flow demand, the downstream pressure isn’t 100 psig but will deviate (offset) below 100 psig. The amount of this deviation usually represent in % of its original set pressure. So, why it is stated that pressure regulator valve will always have this kind of droop? What actually happen and how to overcome this phenomena?
Below illustration will give us a clear understanding regarding the droop phenomena at pressure regulator valves.
Let’s say we have a 2 inch pressure regulator valve maintain downstream pressure with the following characteristic.
- Rated stem travel (from open to close) = 1.5 inch
- Spring coefficient (k) = 150 lbf/inch
- Diaphragm effective area (A) = 20 sq in.
- Set screw adjusted so that the spring is compressed by 0.5 inch.
Second condition, the flow demand is high (or in its maximum design flow). At this condition, the downstream pressure of the valve is down below the set point. Let’s say as per Cv calculation, the high flow demand is designed so that the stem travel doesn’t exceed 50% of its rated travel which is 0.75 inch. At this stem travel, the downstream pressure will be (150 lbf/inch * (0.75+0.5) inch) / 20 sq in = 9.375 psi.
From this 2 different condition, we can conclude that at high flow demand the downstream pressure of the valve will be 9.375 psi. This valve has (15 psi – 9.375 psi) / 15 psi * 100% = 37.5% droop or offset. The droop is unavoidable due to utilize of spring. In our example, we made assumption that the diaphragm effective area is constant but in fact it doesn’t constant. Sometimes, when the valve is closed and the spring is fully compressed, the diaphragm has it smallest effective area. Otherwise, when the valve is open and the spring is relaxed, the diaphragm has it largest effective are.
From our previous example let’s say that at first condition the diaphragm effective area is 20 sq in while at second condition the diaphragm effective area is 25 sq in. At first condition the downstream pressure of the valve is (150 lbf/inch*(1.5+0.5)inch)/20 sq in = 15 lbf/sq in or 15 psi. At second condition the downstream pressure of the valve is (150 lbf/inch * (0.75+0.5) inch) / 25 sq in = 7.5 lbf/sq in or 7.5 psi. Thus the valve droop will be (15 psi – 7.5 psi) / 15 psi * 100% = 50%. We see that the increase of diaphragm effective area will also increase the valve droop significantly.
Since the droop is inherent characteristic of the pressure regulator valves, we can’t make it zero. What we can do is only minimize this droop effect so that the downstream pressure variation is on the acceptable range. Lets say from our previous example, we are required to have pressure regulator valve that has droop not more than 30% of its set pressure. To get the solution regarding this matter, we should do a reverse calculation.
The droop are 30% so that the downstream pressure at high flow demand shall not lower than (15 psi – (30%*15 psi)) = 10.5 psi. From above example we can see that the stem travel variation affect the valve droop significantly. By minimize the variation of this stem travel the droop can be minimizing significantly. At our previous example the rated stem travel is 1.5 inch, let’s change this so that it has rated stem travel 0.5 inch (from open to close). With this stem travel we need to make the spring constant larger so that we can maintain the downstream pressure at 15 psi. The spring constant will be (20 sq in * 15 lbf/sq in) / (0.5+0.5 inch) = 300 lbf/inch. With this new data the downstream pressure at high flow demand will be (300 lbf/inch*(0.25+0.5 inch) / 20 sq in = 11.25 psi. which is 25% of its set pressure or still lower than 30%. Thus we are already overcome the problem.
To reduce the effect of the change in diaphragm effective area while in fully compressed (valve close) and relaxed (valve open), we can make the diaphragm effective are large enough so that the change percentage are quiet small.
Droop or offset at pressure regulator valve can’t be avoided but can be reduced. To reduce the pressure regulator droop we can make the spring rate bigger and rated valve stem travel smaller. To minimize the effect of change in diaphragm effective area, we can make it larger so that the percentage of its change is quiet small.