Two-Way PD Flow Measurement of LPG

March 15, 2011
Is It Possible to Use the PD Meter to Measure the LPG Flow in Two Directions?

"Ask the Experts" is moderated by Béla Lipták (, process control consultant and editor of the three-volume Instrument Engineer's Handbook (IEH). He is now recruiting new contributers for the 5th edition. If you would like to contribute or if you have a question for our experts, please contact him at [email protected].

Q: We are an engineering firm specialized in the oil and gas field. We have been asked by a client to install a PD meter on a liquefied petroleum gas (LPG) feed line to a spherical storage tank. The client needs a two-way PD flowmeter. I have researched the possibility of using the PD as a two-way flowmeter, but I didn't find a clear answer.

Could you please help me? Is it possible to use the PD meter to measure the LPG flow in two directions? If the answer is yes, what are the constraints to be applied? If the answer is no, then what is the best type of flowmeter to use to measure the flow in two ways? Also, what about the temperature effect on the flow measuring accuracy?

Ragab Abdel Fattah
Senior Instrument Engineer
[email protected]

A: A far better solution would be to use a bi-directional turbine meter, which I have personally used on LPG.   The bearing materials need careful selection to ensure repeatability is maintained over time.  Traceability and overall uncertainty are easily demonstrable if custody transfer rules apply.  

The meters I used were 7000 series ITT Barton Turbine meters that were modified and sold in the U.K. by GEC-Elliott Automation.

Richard Furness
ISA 1995 Fellow
[email protected]

A: In the table below, the features of four flowmeter types that are used in bidirectional LPG custody transfer applications are listed as differential pressure type flowmeters, while those used in multiple parallel run designs are not listed due their low accuracy and high pressure drop.

Béla Lipták

A: You can also use ultrasonic multipath flowmeters and Coriolis flowmeters. At least one manufacturer provides bidirectional calibration for Coriolis mass flow- meters. Both types of meter are commonly used for custody transfer applications for LPG and other compressed gases, as well as most liquids.

Walt Boyes, F.ISA
Editor in Chief, control
[email protected]

A: I have seen installations of F&P meters in custody transfer applications for both LPG and liquid anhydrous ammonia. The big problem we experienced- was the low-temperature effect on the bearings, poor lubrication, and the resulting frequent failures.

The installation of turbine meters with special bearing materials (ceramic) for bidirectional measurement and in combination with a prover system successfully operated  in a Santa Fe pipeline system for over 25 years. As you would expect, resolution and accuracy is critical in large-quantity custody transfers. The required resolution was established by the turbine meters, and was proven accurate, often to the third decimal.

I found that old-time engineers, after evaluation of Coriolis  meters, refused to accept them because of insufficient resolution and accuracy.

[email protected]

Q: I took advantage of peaceful holidays to catch up on many items, including reading your excellent blog and article on the Post-Oil World (see Your description of solar-powered hydrogen generation seems logical and inevitable.

There have been many news stories in past year on discoveries of large natural gas fields in the United States, Israel and other parts of the world, and now there is conjecture that these will change the course of energy production for the next decade or so. 

Do you have any thoughts on how, or if, these discoveries will alter the future of energy production? Does this merely delay the hydrogen-solar onset, or does it represent a more important change?

Stephen Freitas
Director Fossil Power R&D
 [email protected]

A: The proved NG reserves of the planet amount to 6,500 trillion cubic feet (TCF). The yearly global consumption of NG is about 175 TCF. Therefore, the proved reserves correspond to less than 40 years of consumption at the present rate, and even less if it rises. The recent discovery of NG in Israel (16 TCF) would supply the planet for about one month. The total amount of NG in the American shale could be 4,000 TCF, but hydraulic fracturing is expensive and dangerous, because it can inject poisons into the groundwaters, and can trigger earthquakes. Whether used or not, the above numbers show that the NG deposits will be exhausted sooner than coal or uranium and about the same time as oil.

NG is very cheap today in the United States—it costs twice as much in Europe—and for that reason, many new pipelines are being built. In addition, there is an intensive advertising campaign in progress promoting NG  as a "clean alternative" to oil or coal. This campaign is misleading (see Table 2), because while the combustion of NG releases fewer particulates, sulphur or nitrogen oxides on a per BTU basis, NG releases similar amounts of CO2 to coal or oil. Therefore, mostly due to low cost, during the next decade, global NG consumption will probably double, unless hydraulic fracturing causes a major accident, similar to the scale of Chernobyl or the BP spill, but even such an accident will not change the trend. For example, last September, when an NG pipe exploded in San Bruno, Calif., killing nine people and destroying 40 homes, it barely made the evening news.

The bottom line is that money spent on "scraping the bottom of the fossil fuel barrel" for NG, coal or oil (or spending trillions on energy wars) will just soak up investment that is needed to convert our energy-economy to a permanent, stable and inexhaustible solar-hydrogen economy.

Béla Lipták