• A combination of liquid, gas and solids.
• Subject to large density changes, but this can be mitigated by installing an independent density meter.
• Thixotropic, where its viscosity changes significantly under different operating conditions.
• Electrically conductive or non-conductive. 
• Operating in the laminar, transitional and turbulent flow regimes.
  

Measuring the mass flow of drilling mud presents various flowmeter technology constraints including:

• Coriolis mass flowmeters are subject to erosion, coating and trapping large cuttings aside from being limited in size. 
• Differential pressure flowmeter primaries exhibit poor accuracy because their operation can be linear, unknown or quadratic in the different flow regimes, respectively. Measurement equations depend on density, viscosity and Reynolds number, which is itself dependent on flow, density and viscosity. 
• Magnetic flowmeters cannot measure non-conductive liquids, and liners are subject to wear. 
• Positive displacement flowmeters are not appropriate to measure liquids with rocks, sand and other debris. 
• Ultrasonic flowmeters are affected by varying density, not accurate in all flow regimes, affected by varying speed of sound, and often not reliable due to poor acoustic conductivity. 
• Variable area flowmeters have moving parts, are subject to plugging, and are not accurate in all flow regimes. 
• Vortex shedding flowmeters cease to operate in the laminar and transitional flow regimes.
  
  Excerpted from Measuring Difficult Flow Streams and More Accurate Flow Control Can Improve Oil and Gas Well Profitability in Processing magazine.

Recognizing that the minimum Reynolds number constraint has been violated, the goal should be to increase Reynolds number such that Reynolds number remains above its constraint at all times, lest the flowmeter drop out and measure zero flow. It should be noted that many (and often all) of these remedies may not be viable, leaving the user to replace the existing flowmeter with another flowmeter that has different constraints and/or that uses a different technology.

• Decreasing pipe size will increase the velocity through the flowmeter and increase Reynolds number. This can increase Reynolds number dramatically for small pipe sizes however its affect can be relatively small in larger pipe sizes. This remedy is often not viable because changing pipe size can be expensive after installation.

• Increasing the temperature of the liquid will decrease its viscosity and increase Reynolds number. This approach is sometimes viable, especially when the flowmeter can be relocated to a different part of the process where the liquid is already warmer.

• Changing liquid composition such as diluting the liquid can reduce its viscosity and increase Reynolds number. Direct dilution of the liquid is typically not viable however it may be possible to relocate the flowmeter to a different part of the process where the liquid is already more dilute and less viscous.

Can you think of another approach to remedy this problem?

 This article originally appeared in Flow Control magazine.

The performance of most flowmeters can be adversely affected by a distorted velocity profile at the flowmeter inlet. One method to improve the velocity profile at the inlet of the flowmeter is to install straight run upstream that allows velocity profile distortions to dissipate prior to entering the flowmeter. To achieve this end, the upstream straight run should not contain any obstructions to flow.

Elbows, tees and throttling valves present an obstruction to the fluid flow and can distort the velocity profile. Because the objective of straight run is the opposite --- reduce velocity profile distortion --- elbows, tees and throttling valves should not be located in the upstream straight run. Answers A, B and E are not correct.

Although designed without turns or bends, fully open reduced-port ball valves do present a small obstruction to flow and should not be located in the upstream straight run. Answer D is not correct.

A fully open full-port ball valve should create minimal (if any) distortion of the velocity profile if proper gaskets are used, if the valve is properly aligned in the pipe, and if the valve is maintained in its fully open position. In a practical sense, this valve could be located within the upstream straight run, but it would be better if it were located upstream of the straight run. However, this valve would not be acceptable in the straight run when the flowmeter installation standard requires the upstream straight run to have a machined finish. Depending on application, Answer C or F might be correct.

Additional Complicating Factors

Fully open ball valves could be pragmatically used in the upstream straight run such as when space is limited or when extensive piping modifications are necessary to install sufficient straight run. Full-port ball valves are preferred but reduced-port valves could be used in a pinch.

This article originally appeared in Flow Control magazine.