"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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Q: In your articles, you have discussed "herding control" as a valuable tool in multivariable safety and optimization applications. Is that strategy a theoretical concept, or did you actually design such systems? Have you used herding control in actual applications?

H. Crowney

A: Yes, I did use the "Puli" algorithm to optimize several combustion processes, keeping a number of controlled variables (CO, HC, excess O2, temperature, opacity) within safe limits inside a "control envelope." I also used it in a computer chip manufacturing application, and once in a building optimization one.

My best known application was the optimization of the IBM headquarters building at 590 Madison Avenue in New York City, where I optimized the energy consumption by "herding" the heat generated in the interior offices (offices without windows that require cooling even in the winter) to the perimeter offices that have windows, and, therefore, require heating in the winter.

Bela Liptak

Q: I have two questions. We have an existing liquefied petroleum gas (LPG) spherical tank. On it, there is an external-cage, low-level switch. Some modifications are going on, and we have to install an additional level transmitter in parallel with the existing level switch.

I'm asking about the suitability of using a differential pressure cell pressure transmitter and what precautions should be considered to get accurate readings? Shall we use a diaphragm seal or impulse lines filled with a special liquid?

What other choice can we use instead of using a differential pressure cell pressure transmitter?

My second question is this: We have existing LPG spherical tanks. We are going to implement some modifications to the inlet/outlet pipeline on these LPG tanks. One shutdown valve will be installed in the inlet line and one in the outlet line. The client asked us to use (if applicable) shutdown valves with self-detecting pressure drop to close this valve in case of line rupture.

According to my experience, we have to install differential pressure transmitters across the shutdown valve to detect the pressure drop in case of line rupture, but the use of differential pressure measurement requires too much straight line upstream and downstream of the shutdown valves, and in this old plant we do not have it. The client asked us to make survey in the market to find a suitable solution for this problem.

I'm asking about a shutdown valve with the capability to self-detect the pressure drop/loss inside the fluid line (LPG). Is there a pneumatically actuated shutdown valve with self-detecting pressure drop? If yes, what is the vendor name and contacts? Is it proved in the LPG application? What are some of the other suitable solutions for this requirement?

l Ragab Abdel Fattah

A: To answer your first question, from the point of view of reliability and availability, my preference is to use a radar gauge. However this would require a large nozzle on the top of the tank and would cost more.

If you decide to use a differential pressure transmitter, you have the following options:

  1. Fill the low pressure connection with a glycol to seal it.
  2. Use chemical seals with capillary connections to the differential pressure cell.
  3. Check with Emerson on the applicability of their remote sensor (3051S ERS). I have not tried it so far.

However it might overcome some of the issues with above options.

In the matter of your second question, since you are installing a differential pressure transmitter for determining the pressure difference, but are not measuring flow, there is no requirement for a straight pipe run. The only thing you should do is study where to install the low-pressure tapping and what setting to select to alarm a leak. Your hydraulic guy can advise this.

Harvindar Gambhir

A: I suggest using a magnetostrictive level sensor, such as manufactured by MTS. It would have a float that floats on the liquid phase of the LPG.  The float incorporates a set of magnets, and the sensor detects the position of the magnets within about 0.0005 inch.  [Note from Béla Lipták: the position of the magnets might be detected accurately, although not that accurately, but the relationship between the magnets position and the level is much less accurate, because that is also a function of the vapor and liquid densities, which vary with temperature and pressure.]

Dave Nyce

A: Differential pressure measurement is proportional to level, if the densities of both the liquid phases are considered: (density liquid – density vapor). Otherwise, any condensation in the nominally "dry" leg will give a zero error.

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