Piezometers for Flow Measurement; Level Gauges

June 29, 2009
How Is a Piezometer Ring Used to Measure Flow?
This article was printed in CONTROL's June 2009 edition.

“Ask the Experts” is moderated by Béla Lipták, process control consultant and editor of the Instrument Engineer’s Handbook (IEH). Preparation of the next edition of Volume 3 “Process Software and Networks” is in progress. If you are qualified to update an existing or to prepare a new chapter or would like to participate in this column, write to Béla at [email protected].

Q: Please explain how a piezometer ring is used to measure flow by the use of a DP type transmitter connected to a rectangular Venturi. My application is to detect the flow on the suction and discharge of a furnace FD fan.

Foster Wheeler India Pvt. Ltd

A: The rectangular Venturi tubes are used in ductwork. If a rectangular Venturi is substantially square, it’s customary to converge-diverge all four sides with the same angles as the angles used for the circular form. Where duct width is different from its height, the short sides are kept parallel with the long sides converging-diverging. A converging angle of 21º degrees and a diverging angle of 15º give satisfactory performance.

The pressure taps are located one-quarter to one-half of the duct height upstream of the inlet and at the middle of the throat section. A piezometer can be used to improve the accuracy of the differential pressure measurement. The piezometer consists of several holes in the plane of the tap locations. Each set of holes is connected to obtain their average pressure.

Venturis with piezometer connections can’t be purged because the purging media tends to short-circuit to the nearest tap holes. Therefore, when it is necessary to protect from plugging because the process gas or fluid is dirty, sealed sensors are used at the piezometer taps. These mount flush with the inside wall of the duct. In such cases, each pressure sensor functions as an independent measuring device, and these measurements are combined to read the differential pressure, while automatic compensation is provided to correct for static pressure variations as the differential is converted into volumetric flow.

Piezometer connections are normally used only on very large ducts or where the most accurate average pressure is desired to compensate for the variations in the hydraulic profile of the flowing gas or fluid.

Béla Lipták 
[email protected]

Q: Can you provide me some international standards guidelines to finalize the quantities of F&G devices in the flare area? I referred BS6959, API 14C, NFPA 72 and couldn’t find any guidelines for the detector quantities.

Suby George
Mahatta Trading Company, Kuwait

A: The issue of performance-based fire and gas detection has been taken up by the ISA84 committee, which has written the bulk of the technical reports surrounding the implementation of the IEC 61511 standard for safety instrumented systems. The subject of implementing a fire and gas detection and suppression system in conformance with IEC 61511 is the subject of a new technical report. This technical report covers subjects such as the quantity and placement of detectors in order to achieve adequate coverage of fire and gas events. While the technical report is still in draft form, the techniques presented in the report should provide a mechanism on how to answer the questions posed. The report from the ISA will be printed sometime in the next few months.

Edward M. Marszal, PE, CFSE
[email protected]

Figure 1. Magnetic gauge mounted to process vessel. A float inside triggers magnetic flags to indicate level.

Q: I am an academic chemist with an interest in performing solvent-free reactions by grinding together solid reactants in ball mills. I came across your articles on distillation control and optimization while trying to assess the potential energy savings to industry in moving over largely solvent-free processes. While it seems clear that distillation is energy-intensive, I am finding it difficult to find figures for the industry as a whole. I would greatly appreciate your advice on whether such figures might exist, and if so where I might find them.

Dr.Stuart James
Queen’s University Belfast
[email protected]

A: A presentation on the thermodynamic efficiency of distillation appears on pp. 54-59 of my book Distillation Control for Productivity and Energy Conservation, 2nd ed., McGraw-Hill, New York, 1984.

F. G. Shinskey
[email protected]

Q: We are using ordinary level gauges in the inlet separator (offshore) to measure the level. Within a matter of hours, the glass becomes dirty and we can’t see anything. Could you please suggest a solution for this problem?

Rajeshkannan .N
[email protected]

Figure 2. A magnetostrictive transducer for mounting on vessel.

A: I recommend replacing the sight glass with a magnetic gauge that uses a float with an internal magnet, and have magnet-operated indicators. Such a gauge is shown in  Figure 1. Another alternative, if they want to add an electronic output, is using a magnetostrictive gauge (Figure 2). Either solution would avoid problems due to the optical clarity of the liquid.

David S. Nyce
[email protected]

Q: I’m wading through the Fisher Handbook on control valve sizing, and I have some questions. The maximum valve sizing coefficient on one page is stated as Cvmax= 51 for a 2 in. globe valve and on another page as  Cvmax= 47. Is this a typo? Also, the required valve sizing coefficient is stated to be Cvrequired = 0.85Cvmax. Is Cvmax the value when the valve is 100% open, and Cvrequired the recommended maximum at which the valve should operate?

Paul Smith
[email protected]

A: The Cv of a single-ported, equal percentage globe valve with flow under the plug and a maximum trim (1.62 in.) has a Cv of 47. So, if that is the valve you have, it will have a Cv of 47 when fully (100%) open. Naturally, if the valve is double ported or if the flow is over the plug, the Cv changes. As to the required Cv, I tend to be more conservative. I like to have the normal valve opening to be between 50% and 75% of maximum. In my experience, 85% does not give us enough upside safety margin.    

Bela Liptak