Pressure Loss in Venturi Meters: How much line pressure/energy does the PFS HVT modified short form Venturi actually consume?

History of Venturi Meters

Venturi meters are well-defined in literature with a variety of examples, such as the Classical or Herschel-type Venturi. The length of these meters was quite long because their convergent and divergent angles were small and the throat length of these meters are equal to the throat diameter. In most examples, the converging cone does not influence the meter’s coefficient; however, it does have an effect on the overall energy consumption of the meter.

The Classical design also contained an averaging pressure sensing ring (called an annulus or annular chamber) at the high and low-pressure sensing cross sections. Depending on the manufacturer or model of the classical meter, the basic design included an annular ring at both the inlet – high-pressure area, and throat low-pressure locations or, in some cases the annular ring was located at only the low-pressure position.

For cast and ductile iron examples, the annular chamber was cast into the body, and depending on the vintage, manufacturer, and model, multiple internal taps led to the annular chamber. This “averaged” pressure was then led to a single tap externally which was used for connection to the secondary instrumentation system. To this end, the meter was designed to be used on clean, non-corrosive fluids eliminating the concern for plugging the chamber which would have resulted in the partial or total loss of the pressure signal to the secondary instrument. In addition, for the cast and ductile iron models, the annular averaging chamber was cast into the meter body and could not be easily coated or cleaned. Note, that for fabricated-type meter designs where there was no cast geometry, if one wanted an averaging annular ring, it was located externally via a tubing ring or external manifold that was connected to 4, 6, or 8 internal taps.

Updated Venturi Flow Meter Designs

The design and materials used in Venturi meters have not remained static over the last century, and although the fundamental principles have remained the same, some important updates have made today’s technology even more accurate and reliable than past versions.

There are two versions in particular that have made significant impacts on Venturi metering – the modified short form Venturi meter and the insert Venturi meter.

Version 1: Modified Short Form Venturi Meters

Modified short form Venturi meters such as the PFS-HVT are more tolerant of upstream conditions. These meters have no downstream straight pipe requirements for standard accuracy, have lower head loss, are less susceptible to blockages and plugging as the annular chamber was replaced with a single pressure tap which can be easily cleaned should any plugging occur, and they are more accurate than most other meters. 

Typical head loss percentages for modified short form meters range from 3.0% to 10.0% of differential depending on Beta ratio selected, which is better than the Classic Venturi.For example, if the maximum flow rate through the venturi meter with a full recovery cone configuration resulted in a 100.0” differential and the beta ratio of the meter (beta ratio = d/D) was 0.7, the unrecovered energy or head loss would be 3.5” or about .13 psi.  As a practical matter, this would mean that if the line pressure upstream of the Venturi meter was 10.0 psi, the line pressure on the discharge side of the meter would be 9.87 psi.

The overall length of the HVT is also considerably shorter than the Classic Venturi because the intersecting angles are much greater and the discharge end of the recovery cone does not end in the full downstream pipe line size, but rather, is truncated or cut short, while the flange is designed to mate directly to the downstream pipe flange.

This allows the device to be shorter, hence the name, and more energy efficient since the frictional loss through the long Classical Venturi meter recovery cone consumes a great amount of energy. In addition, the HVT throat section is about one-half the length of the Classic Venturi meter throat section which also contributes to lower frictional loss. It is interesting to note that most major independent laboratory flow calibration facilities around the world use Venturi meters as their primary standard. As well, pump and blower manufacturers also use Venturi meters and other differential-type devices as efficiency testing standards because of the high accuracy, repeatability and long life expectancy of the meter.

Version 2: Insert Venturi Meters

A useful addition to the world of Venturi metering is the insert Venturi meter. This design has the same geometry as the full flanged design but the profile is entirely inside the pipeline and is held in place with a center holder that sandwiches between two existing pipe flanges, much like an orifice plate.  

The “laying length” of the meter is the thickness of the center flange so there is a considerable saving of space.

But the benefit of this design is that it utilizes a static low-pressure throat tap. Unlike a “true Venturi meter” its high-pressure sensation comes from a corner location on the inlet side of the center flange. As previously explained, the Classic Venturi senses true static pressure at both the high and the low-pressure tap cross sections, as does the modified short form, pressure vessel type Venturi design.

In order to claim that a tap senses “true static pressure” the tap must sense pressure perpendicular to the axis of the flowing line fluid. The Classic and short form Venturi meters do that very successfully in the flanged design, but without an upstream section available to locate the high-pressure tap, the insert type Venturi meters develop its high-pressure at the tap that is effectively “looking’ at the on-coming flow, which makes the insert type Venturi meter somewhat more sensitive to the effects of upstream non-straight piping. The basic accuracy of these meters is identical to the flanged design at +/-0.50% of the actual rate of flow. However, the PFS static high-pressure tapped insert Venturi meter design moved the high-pressure tap off of the center flange and into the upstream spool piece. With this adjustment, the meter enjoys the same installation insensitivity as the traditional flanged, full body Venturi design at the proper static pressure sensing location.

Conclusion

Venturi meters are useful devices to measure liquids, gas, steam, slurries, cryogenic fluids, and mixed media flows. They are versatile in that their laying length can be changed to fit a defined space; they can be modified to provide rate of flow control or measure sewage; they can be used reliably for billing or custody transfer; and they can be used for rectangular or circular metering. In addition, Venturi meters can be oriented in any plane and can measure accurately whether the line fluid is flowing upwards or downwards. Venturi meters are not subject to downstream piping effects, only upstream piping, the effects of which are well-known and predictable.

One other significant advantage is that the HVT is an ideal “primary” device when the minimum to maximum expected flow rate range is wide (50 or 100 to 1 on flow).  PFS can provide an integrated system accuracy profile that would utilize the model and number of DP transmitters required to accurately satisfy most wide-range applications.

Before you eliminate the use of a Venturi meter over concerns about the amount of “energy” that the device uses, contact PFS for a factual answer to this question.