Flow is probably the most important measurement in the process industries. The ability to measure and control the flow of materials through the various parts of a process is critical for most all unit operations. Good flow measurement is essential for mass and energy balances and other calculations used to evaluate process performance.
Differential pressure based flow elements are still the primary mechanism for flow measurement in most locations. The basic technology behind DP flow has been around for over a century. It has been around for so long that we sometimes take it for granted. There are many places where this well-known technology can go wrong and give us less than satisfactory results.
During a recent process control audit of our diesel hydrotreater unit, I decided to verify all of the orifice calculations and DP transmitter configurations in the unit. I was unpleasantly surprised at the results. They were a perfect example of many of the things that can go wrong in the design, procurement, installation, commissioning and maintenance of a flow measurement device. It was a good reminder of the things that we should all be aware of in the life cycle of a DP flow device. In the sections below, I'll share what I found and what we can all learn from it.
Anyone Can Size an Orifice, Right?
There are hundreds of programs available to do the basic calculations required to size a DP flow measurement device. All of them use the same basic equations and will generally give the same results. The problem is with the data that goes into the calculations. When I reviewed the calculations made by the E&C firm that built the unit 9 years ago, I found two fundamental flaws that created significant errors in about 75% of the meters in the unit.
Our refinery sits at an elevation of 3200 ft. That means that our atmospheric pressure is not 14.7 psia but 13.1 psia. We always use 13.1 psia to convert to absolute pressure. The E&C firm that did the calculations used the 13.1 value as requested. However they also used 13.1 instead of 14.7 as the pressure value in the definition of “standard” conditions. It sounds like a small thing but it resulted in an error of about 15% in all of our gas flows that were referenced back to standard conditions.
Refineries in the US typically measure hydrocarbon flows in Barrels per Day (BPD). These volumetric measurements are converted back to the density of the fluid at 60 degrees F. This allows flows of the same material to be compared without having to compensate for temperature. The original E&C firm calculations were done in GPM at flowing conditions and were not corrected back to standard conditions. When the ranges were configured in the DCS they were converted to BPD but were not corrected then either. This resulted in errors of a few percent to almost 80% depending on how far away the flowing temperature was from 60 degrees F.
These two simple mistakes resulted in bad flow measurement indications on about 75% of the meters in the unit that were not identified for almost 9 years.
This was not the first time that we have found errors in orifice calculations done by others. A couple of years ago we had to replace a 6" stainless steel venturi two weeks before a turnaround when we discovered that someone had misplaced a decimal point in the gas density used in the calculation.
Orifice calculations are simple, right? We assume that anyone can do them and we generally accept the results without question. However, this practice will lead to errors that may cause problems during startup, or worse yet, go undetected for years.
In Part 2, I'll discuss other things that can go wrong during procurement, startup and ongoing maintenance of a DP flow meter.
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