Rehabilitated Differential Pressure Meters: Expected Accuracy After Rehabilitation

During the past 15 years, PFS has provided field and factory meter rehabilitation services for a wide range of differential primary elements including venturi meters, flow tubes and orifice meters. In order to provide appropriate guidance to those users who are considering whether to replace or rehabilitate an existing meter, PFS has performed a wide range of factory and laboratory tests which allow us to qualify what can be expected when considering a rehabilitation program.

The ideal rehabilitation program is one where the meter is rehabilitated in our factory primarily because, while a field rehabilitation can certainly be done, the execution of the various steps in the rehabilitation process requires equipment and services that are difficult to duplicate on the field. This analysis provides guidance for both a factory and a field rehabilitation process. Please see PFS Rehabilitation Specification Guide for information on the two options.

Basic Differential Pressure Meter Accuracy:

Generally speaking, there is no accuracy difference between a field or factory rehabilitation process in terms of the basic accuracy of the primary element. The process that is used in both cases is as follows:

  1. Identify if the as-found meter is a candidate for rehabilitation which involves an internal inspection of the meter’s body condition, the throat condition and the pressure tap condition. Once determined by inspection that the meter is a candidate, the following applies.
  2. In both the factory and field rehabilitation process, the goal is to remove any unwanted matter that may have built up on the internal section of the meter so that the result is that the interior profile is brought back to its original geometry. The difference between factory and field rehabilitation in this case essentially is that the equipment for sandblasting (which is the process that is used to remove unwanted matter) is more effectively available in a factory environment as compared to a field environment.
  3. By removing all unwanted matter, the true base metal surface is exposed and, in the case where there has been erosion of the base metal, there are a variety of NSF-approved filler materials that can be used to return the profile to its original tolerance with proper finishing of the applied metal filler.
  4. This same process is applied to the throat section except that the throat on meters prior to about 1980 were made of bronze which means that during the rehabilitation process (field or factory) the bronze material is isolated and not sandblasted. The bronze liner is cleaned with a bronze metal cleaner and the low-pressure tap is inspected to be certain that it has the proper edge geometry. If necessary, re-dressing of the tap may be required, which can be done in the field but may be easier to accomplish in the factory.
  5. Greater attention is required to the inlet through throat sections of the meter since those sections control the accuracy of the meter.
  6. Once the surface has been properly treated, the NSF-61 coating process, which includes a primer and several finish coats of approved epoxy paint, results in an interior meter surface that is acceptable as a flow measurement product.
  7. The critical internal cross sections are then accurately measured at 6-8 positions and a new design flow calculation is provided which utilizes the discharge coefficient that applies to the specific device being rehabilitated but the flow calculation and rate vs differential information is based on as-rehabilitated dimensions which may differ from the original dimensions.

Conclusions:

If the original meter design had a proven accuracy of +/-1.0% of actual rate of flow, the rehabilitation process and revised design flow calculation will bring the meter’s pre-rehabilitation accuracy back to its original accuracy.

The life expectancy of the meter, all things being equal, will become that of the original meter. Thus, if the original meter lasted 40 years; the rehabilitated meter will last somewhat longer, potentially, due to the use of better coating processes but, in every case, its useful life expectancy will not be less than its original life expectancy.

Installed Metering System Accuracy:

After rehabilitating hundreds of meters and inspecting thousands of meters over the past 30 years, several important points can be raised:

First, there is a difference between “based meter accuracy” and “installed metering system accuracy” which is:

Basic meter accuracy is the accuracy of the basic design of the meter as proven by the designer and original manufacturer of the meter. Ergo: if a Badger PMT meter had an originally claimed accuracy of +/-1.0% and it is rehabilitated to its original condition – its rehabilitated accuracy will also be +/-1.0%.

Installed metering system accuracy goes beyond just the basic meter accuracy and brings into the analysis all of the external elements that impact the indicated and totalized flow data that is produced by the metering system which includes:

  1. Basic meter accuracy.
  2. DP transmitter accuracy as a function of type of DP transmitter, model of the transmitter, range of the transmitter and span for which the transmitter is being used.
  3. Impulse tubing geometry, size and orientation.
  4. Inlet pipe to meter ID mismatch.
  5. Upstream disturbers and their impact on the installed accuracy of the meter.

Conclusions:

While the rehabilitation process used on the meter will not result in any improvement to its original accuracy, because PFS investigates and makes recommendations concerning points (b, c, d and e)above – in fact, it is very likely that the result will be an improved understanding of the “installed system accuracy” because our analysis will take into consideration important accuracy affecting elements such as improper impulse tubing conditions, upstream disturbers which were never addressed during the original installation, DP transmitter performance that renders the electronic processing of the differential pressure inaccurate because of improper calibration of the transmitter or over range operation etc.

At the conclusion of the rehabilitation process, the owner is provided with a new O&M manual which accurately reflects any changes to the interior dimensions, a new rate vs differential table and an updated differential flow calculation document.

Learn more about our meter rehabilitation services, or contact PFS to connect with one of our experts.