Straight Talk

Jan. 4, 2008
Greg McMillan and Stan Weiner Bring Their Wits and More Than 66 Years of Process Control Experience to Bear on Your Questions, Comments and Problems
By Greg Mcmillan and Stan Weiner, PE

Stan: We need to have some straight talk on the piping requirements for desuperheaters introduced in our year-end puzzler bonanza last month. But first, Sonya Gibson sent in the correct downstream pipe length requirements of sensors for static mixers and heat exchangers. She rightly points out that the thermowell or electrode should be at least 20 diameters downstream of a static mixer or heat exchanger to provide blending of the exit stream.

Greg: Clinton Anderson has offered to provide us with a more comprehensive view of the installation requirements for desuperheaters.

Clinton: The piping length requirement depends on a number of variables, but there are some simple rules of thumb for the straight piping length (SPL) to the first elbow and the total sensor length (TSL). Actual SPL and TSL values depend on the quantity of water required with respect to the steam flow rate, the temperature differential between the water and steam, the water temperature, pipe diameter, steam velocity, model type, etc. They are computed by software programs. SPL (feet) = Inlet steam velocity (ft/s) * 0.1 (s). TSL (feet) = Inlet steam velocity (ft/s) * 0.2 (s).

Greg: It takes about 0.1 seconds for the water droplets to be vaporized and another 0.1 seconds for them to be mixed. What are some typical steam velocities?

Clinton: Typical values for the inlet steam velocity, upstream of the desuperheater, range from 25 ft/s to 350 ft/s. Below 25 ft/s, there is not enough motive force to keep the water suspended in the steam flow. Water tends to fall out and run down the pipe to a drain. When this happens, the water no longer cools the steam, and the system thinks it needs to add more water, which compounds the problem.  Problems can include pipe-wall erosion and high thermal stress gradients in the pipe wall; i.e., hot top and cold bottom, which can crack welds or warp the pipe to an egg-shaped cross section. Higher velocities cause the desuperheater to vibrate, which can fatigue the unit to the point where it breaks apart.

Greg: What are some things  to look out for besides velocities that are too low or high?

Clinton:  “Does your desuperheater drool like a new born baby?  Check the spray water control valve for leakage.” The desuperheater is often blamed for poor performance when it is the control valve that is leaking. The water falls into the line in large droplets that are not easily absorbed. Poorly designed installations also cause poor performance. Engineering firms will often place a desuperheater too close to an upstream flow disruption, such as an elbow, diverging or converging pipe tee, etc. Good desuperheating requires an evenly distributed flow profile.

Greg: What are some approximate turndown ratios for various types?

Clinton: Typical turndowns are 3:1 for fixed orifice nozzles, 10:1 for venturi style desuperheaters, 25:1 for variable orifice nozzles, and 50-100:1 for steam- assisted desuperheaters.

Greg: How about some best practices?


  • Install the appropriate size strainer upstream of the water control valve to prevent plugging of the desuperheater nozzle and damage to the control valve.
  • Inspect equipment regularly looking for nozzle damage or plugging. They can cause the spray pattern to impinge on the interior of the steam line, which may damage the system.
  • Specify controls valves for Class V shutoff so that leakage does not cause unintentional desuperheating.
  • Ensure accurate valve control to minimize wasted energy and guarantee temperature accuracy.

Greg: Now let’s move on to the straight-run requirements of nozzles, orifices, venturis, annubars and vortex meters. Detailed information for differential head meters is in the ANSI 2530 and ASME Fluid Meter recommendations and the API RP 550 Manual for the Installation of Refinery Instruments and Control Systems. In comparison, the straight-run requirements for magnetic flowmeters are minimal and nearly negligible for Coriolis mass flowmeters.

Stan: I can’t believe that engineers are still using differential-type flow measuring devices. They were obsolete more than 20 years ago. Their installed costs and maintenance are high. Their rangeability and accuracy are poor. They require long pipe runs with mean difficult piping arrangements, adding to their installed costs. Their sensing legs freeze up in cold weather if not insulated properly, or the liquid vaporizes when hot.

Greg: Now for some straight talk on the value and check points of process control improvements. Much of the benefit from the application of advanced process control (APC) comes from the improvements made to the basic control system. Start earning the benefits of better basic control now by asking the following questions. (The values listed below are for a 2% required change in controller output, a 1% allowable control error, a process dead time of 10 seconds, a process time constant of 10 seconds, a process gain of 1, and a disturbance time constant of 1 minute.)

  1. Does the control valve quickly respond to the required changes in the controller output (e.g. half deadband and resolution < 0.2% and response time < 5 seconds)?
  2. Is the sensor noise and repeatability error less than 1/10 the allowable control error (e.g. noise and repeatability error < 0.1%)?
  3. Is the lag of the measurement less than 1/10 the process dead time and process time constant (e.g. measurement delay and lag < 1 second)?
  4. Is the execution time of the controller less than 1/5 the process dead time and process time constant (e.g. control module execution time < 2 seconds)?
  5. Is the controller tuned to provide a standard deviation of the controlled variable less than 1/5 the allowable error (e.g. std dev < 0.2%)?

Stan: Now for the good part: The “Top Ten List.”

Top Ten Broken New Years Resolutions

(10) Have more bark than bite. Can I at least growl? Will I be forced to wear an anti-bark collar? A muzzle?
(9) Stop making fun of seniors. Who else do I know?
(8) Stop focusing on dead time. What else is there at Sun City?
(7) Final element resolution resolution. Why should I get unstuck when valves are stuck, and it gives me a chance to repeat words?
(6) Get into hybrids. Can a hybrid face up to a 4-ton high-lift truck with a cattle guard? Can I drive under a cow?
(5) Show my more sensitive side. Wait, that is what I am sitting on.
(4) Stop drinking cheap booze. I will give this another shot.
(3) Listen to hip-hop. What if I am not hip and can’t hop? 
(2) Become rich and famous. How about poor and infamous?
(1) Learn to sell. How can I sell a product when I can’t sell myself?  

This month’s puzzler

Too Much Information?

How much information is in the process variable versus the output of a PID controller on disturbances, sensor drift and offset, and valve deadband and resolution?

Send an e-mail with your answer to the Puzzler, CONTROL questions, or comments to [email protected].