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Apr
19

Valves that can Turn on a Dime, or at Least a Quarter – Part II

In my younger days I was presented with the critical need to make air compressor surge and electrical phosphorous furnace pressure control loops able to handle very abrupt and extreme disturbances. I vastly prefer the present to that present. These applications offered more excitement than engineers should be allowed to have.

The compressors provided the air feed to multiple exothermic reactors whose flow could drop enough on a reactor trip to trigger a surge in about 2 seconds. A surge every month would cause the other reactors to trip and cause accumulating damage and loss of efficiency in the compressor besides reactor downtime and a subsequent challenging startup of reactors and the associated waste oxidation boilers.

The phosphorus furnaces had to deal with what was called “controlled explosions” from sudden shifts in the ore around the electrodes (slag slides) that caused bursts of water vapor and CO2 besides phosphorus vapors and particles. The slag slides caused a pressure spike large enough each shift to blow the water out of the electrode seals. There were tubs of water around the furnaces to jump in if the hot phosphorous landed on you. Little fires would break out when you walked by the furnaces from your shoes scraping the phosphorous residue on the floor.

These were big problems in terms of both size (18 to 24 inch pipelines) and the safety implications besides the process efficiency and capacity considerations.

High speed recorder measurements of the of the compressor flow and furnace pressure response confirmed that the process dead time in both cases was essentially zero and the observed dead time was due entirely to the components in the automation system. I installed transmitters with a sensor response time constant of less than 0.1 seconds and controllers with a special scan rate of 0.05 seconds. I had to take some special precautions in the configuration to insure the controller loading would never have negative free time (a lesson as well for our personal lives).

The control valves were the largest source of dead time. The pre-stroke dead time and stroking time for the big actuators were estimated as the fill or exhaust factor for the actuator supplied by the valve manufacturer divided by effective fill or exhaust flow coefficient of the existing positioner. This yielded pre-stroke dead times ranging from 1.0 to 2.0 seconds and stroking times of about 10 to 20 seconds. A booster had a fill and exhaust flow coefficient that was 10 times larger than the positioner and therefore offered dead times and stroking times that were 10 times faster. However, I knew the actuator connection and air tubing would then become the restricting limitation, so I had these sizes judiciously enlarged in the field, I also added a position transmitter (before the days of Hart and Fieldbus positioners with position read back).

Armed with the rule “boosters instead of positioners should be used on fast loops” and a copy of the theoretical frequency response studies to back it up I arrived onsite for the compressor application and boldly insisted against the advice of the well seasoned instrument maintenance technician to replace the positioner with a booster on the compressor vent valves.

My confidence was shattered the morning the first surge valve was put in service. The flow transmitter showed the impending surge and the controller asked the valve to open. The valve responded by doing the worst possible thing. It slammed shut before the forward flow to any of the reactors had been established.

The technician who wanted the positioner took me to the surge valve and showed that he could move it to any desired position by tugging on the actuator shaft. Obviously, the buffeting action of the turbulent flow could cause the disc to wander and eventually close. The actuator size was checked and found to be adequate; the spring rate was increased but the results remained the same. Subsequent tests showed that the stem resisted movement considerably better if the actuator was fed directly from an I/P transducer and that it could not be budged at all if a positioner was installed.

We still needed speed, so I installed the booster on the outlet of the positioner. Unfortunately the positioner looking into the small inlet port volume of the actuator can change the pressure here much faster than the booster can change the actuator pressure. The consequence is an audibly and visually impressive 1 cps limit cycle. The booster had a built in bypass whose restriction was then adjusted so that the positioner could see part of the actuator volume. Of course, the more you bypassed the booster, the slower the valve got so the restriction was opened just enough to reliably prevent the limit cycle.

On the furnace pressure control application, I put my pre-stroke dead time and stroking time requirements on the control valve specifications along with a test to be witnessed by me at the valve manufacturer. When I arrived at the valve factory, the control valves each had a booster instead of a positioner. I walked up to the valves and showed them how I could stroke these big butterflies by grabbing the shaft. Needless to say they were astonished. The actuator sizing and spring rating was rechecked. We put on the same booster and positioner combination with a tuned bypass and the problem was solved.

There is no official explanation but obviously since neither one of us had the strength to move the shafts of these big boys at will, the booster was doing something to assist us. Possibly the extreme outlet port sensitivity of the booster (fractional inch of water column) provided positive feedback in that a slight change in the diaphragm actuator volume would cause a change in the booster outlet port pressure and hence booster flow.

These valves were designed for throttling with minimal packing and sealing friction so the dead time from deadband and resolution limits were small and in fact less than the booster because the inlet sensitivity of the booster was reduced by design to work better on piston actuators. Thus, the positioner had less dead band than a booster and the combined use of them meant that source of the most of the loop dead time was relegated to actuator pre-stroke dead time. This is not the case for isolation (block and interlock) valves masquerading as control valves so here as promised last week is the help you need for this bigger problem discussed in my upcoming Control Talk column (May 2007 issue of Control magazine).

Top Ten Signs Your Control Valve is an On-Off Valve in Disguise

(10) Valve body looks suspiciously like the block valve next to it

(9) Actuator looks suspiciously like the one on the interlock valve

(8) Process engineer is seen going out to lunch with the on-off valve supplier

(7) The valve deal is a steal

(6) Your flow is on-off

(5) Positioner measures actuator shaft instead of ball or disk stem position for feedback

(4) Limit cycle amplitude exceeds largest data historian compression setting

(3) 360 degree feedback in your loop becomes 360 degree feedback in your performance review

(2) The valve package is nicknamed “Sloppy Joe”

(1) No leakage till the controller output is greater than 40%