"Herding" Control and LPG Problems

A Reader Asks Us if "Herding Control" Is a Theoretical Concept, or a System that Our Very Own Bela Liptak Designed. Also, What Is the Suitability of Using a Differential Pressure Cell Pressure Transmitter.

By Bela Liptak

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Whether LPG "heavies" can condense depends on the ambient temperature swing. A glycol filled "dry" leg is a good solution. I am less enamored of using capillary systems, which can be sensitive to sunlight exposure (try to keep them on the shady side of the tank). Rosemount's system of a HART-coupled LP sensor may be worth exploring.

The magnetostrictive float transmitters can be a good solution, but can be more difficult to install. Note that these have zero error if (liquid density – vapor density) varies significantly, to the point where the float just sinks. The zero error can be as much as 60% of the float length.

Guided-wave radar will work on LPG if selected properly and it's economical, but it really works best inside the vessel, rather than in an external chamber where the LPG may boil due to solar exposure. Thermal insulation can be desirable on external chamber devices of any sort if solar radiation is significant.

Ian H.Gibson
gibs0108@optusnet.com.au

A: In answer to your second question, do this by operating a pneumatic control valve on the basis of pressure drop across it; a lower-maintenance and less expensive solution is to use check valves.   

On the inlet line to the vessel, you need to install a regular check valve "in reverse," meaning that it is fully open when the flow direction is into the tank, but closes as soon as the flow direction reverses. On the discharge line from the tank, you should use an excess flow check valve (EFCV).

The EFCV consists of a plug, a seat and a spring, all housed or supported in a cylindrical tube. The valve connections can be screwed or flanged as shown in Figure 1.

Under normal operating conditions, the force generated by the flowing process fluid is directed against the valve head and attempts to close it, but the spring is stronger and keeps the valve open. If the flow suddenly increases to an excessive rate, the force against the plug and the differential pressure across it overcomes the spring force, and the valve closes. The EFCV does not require up- and downstream straight runs as the velocity profile across it does not matter.

Inside the EFCV, there are one or more bleed ports around the plug, so that after the valve closed, the bleed can equalize the pressures up- and downstream of the plug, so that it can reopen, if the pressures equalize. However, if a pipe break occurs, the differential pressure across the valve will remain constant (the difference between the pressure on the tank and the atmosphere downstream), and the valve will stay closed until the pipe is repaired. It should be remembered that because of the required seepage through these valves (which provides their equalizing feature) they will not give tight shutoff.

Béla Lipták
liptakbela@aol.com

A: You can contact Festo Pneumatics, which can provide the following mechanism/pneumatic hook up: a 3/2 N/O (read 3-way, 2-position, normally open) valve, pneumatic operated by loss of pressure, spring-returned, normal position. The required air supply is 6 bar to above the 3/2 N/O valve, while its outlet port above the 3/2 N/O valve connects to your automatic pneumatic shut-off valve. The port that triggers this valve connects to your rupture line for sensing loss of pneumatic pressure.

To increase safety, you can provide redundancy for logic validation on line rupture.

I found such systems on the Internet, by using the key words: "Shutdown Valves, LPG."  Your problem is interesting. I am not sure if any of these valves have gotten TÜV approval for emergency shutdown based on the SIL of your plant.

Gerald Liu
gerald.liu@shaw.ca

A: Do not worry about the straight line segment. That is a requirement for precision flow measurement, but not for your safety systems application. In fact, you cannot use the d/P information for accurate flow measurement since your valve is not an orifice with known diameter. So just install a conventional low-accuracy (if that is less expensive) D/P transmitter across the shutdown valves. The reading at normal times will be zero flow, and some positive reading in case of valve failure to completely shut off. However, I do not understand how a D/P across the valve can detect line rupture without some type of piping diagram.

Richard H. Caro
RCaro@CMC.us

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