Have you seen a loop more oscillatory in automatic than in manual? Is the problem loop interaction, aggressive controller tuning, or a sticky-sloppy valve? If you have a digital positioner with position readback on a throttle valve supplied by a control valve manufacturer, you have a chance of figuring out if the valve is the culprit. If you don't have digital positioner and a real control valve, the valve probably is the problem.
For rotary valves, the position feedback measurement must be of stem rather than shaft position for the readback. Block or on-off valves provided by piping valve manufacturers often have position feedback on the actuator shaft. Even when the position of the stem is measured, the high friction of the rotary element (ball, plug, or disk) seal may cause shaft windup to the extent where even the stem position is not representative of the internal element position. I have seen this big time for a ball valve posing as a control valve in phosphorous service. Even for a new valve on the bench, the ball did not move for a 10% change in stem position. In these cases, the position feedback and readback are lying and the diagnostics from a smart positioner are misleading.
In the 1980s the slapping of piston actuators and positioners on piping valves became rampant. Since they were less expensive than a control valve, they were already in the piping spec, and the ISA standard for valve response testing had not been published, the process engineers were easy targets. After all, the piping valve worked well as a block and isolation valve, the leakage specs were impressive, and pluggage was often less of problem. Plus you could always blame process variability on mysterious causes since no one knew what the valve was really doing. Position readback was rare since it required a separate position transmitter and wiring. Valve tests by technicians in the field did not reveal a problem because 25% or 50% changes in signals were used. All but the biggest valves or dampers looked decent for such large changes in valve signals. The idea that enormous controller output changes (e.g. 25 to 50% per sec) did not occur except for special situations, such as surge control, didn't seem to cross most minds.
In the next two weeks we will get into the importance of readback in actual control valve scenarios. In the mean time if you want more information on the effect of control valves on loop performance, check out the following articles:
"Improve Process Loops" , Chemical Processing, October, 2007,
"A Fine Time to Break Away from Old Valve Problems" , Control, Nov, 2005
"What's Your Flow Control Valve Telling You?" , Control Design, May 2004
Good article. Will you please write additional about this subject.
I agree it was definitely worth my time at this site, the actual person knows about what he is saying. I hope you come back and post more, when it's convenient.
Greetings from Denmark. Like your blog!
As a inexperienced blogger personally, its beneficial to see a quality blogger for something new. I hope to find out a lot from studying other folks weblogs and I feel that the one you have has quite a bit to offer the website reader, many thanks for finding the time and making the effort. Regards
If you guys have nothing to say about the post topic then don't say anything at all. You people are obviously just spammers. Please refrain from such indecent behavior .
I really loved your post and would like to know if I provide a link back to your blog if I can use your article on one of my website?
Pretty good post. I just stumbled upon your blog and wanted to say that I have really enjoyed reading your blog posts. Any way I'll be subscribing to your feed and I hope you post again soon.
my opinion is always a book should be written on each existence experience...that and should you could integrate humor into your written experiences, would result in a fine book
The blog was absolutely fantastic! Lots of great information and inspiration, both of which we all need!
I started off cigarette smoking when I was in my teens. This turned out to be the most unfortunate mistake I've made. Today I'm older and I've got chronic copd. While looking to quit cigs, I came upon the e cig and i intend to try it out. Ideally, it'll help me with this particular unhealthy addiction.
While seeking to quit cigs, I discovered the e cig. The electronic cigarettes wotks on a nicotine material which consists of just nicotine. Absolutely no toxic chemicals whatsoever. They've luckily saved my life. Stopping inhaling and exhaling toxic chemicals feels good to me!
Great post, I just bookmarked it on Digg.
The Best Premium Tobacco Cigarettes - Try it yourself !
I stopped cold turkey a week ago and it's beginning to make me feel crappy today. My best friend says I won't keep going but I'm so confident about this I won't fail. recently I started having cramps inside my gut and I really am craving a cigarette. I cannot give in since I'm quitting smoking for our kids. I'm grateful to have the electric cigarettes. It really has been a big bonus in my opinion.
I haven't had a drag in 6 years. The toughest thing regarding giving up is the very first few days. Happily this stuff didn't last and things got better day-to-day thanks to the e cigarette. You should not stop. Just simply persevere and you will definitely realize success once and for all.
I have not had a smoke in for many years. The most horrible part concerning stopping smoking would be the initial couple weeks. Fortunetly that part went away quickly and stuff improved as time went on because of the e cig. Never stop trying. Just simply be strong and you inevitably you will be a non-smoker permanently.
I had been doing silk n applications for at least a year at this point on my mustache. Before this I had an extremely thick dark beard. I am proud of the final results. The actual hairs are really thin and I have not gone back for treatment for approximately 6 months. I am going to mention that it did hurt just a bit on my top lip and there were definitely times where I had a little bit of redness on my neck. I would recommend Intense pulsed light to people having heavy hairs or currently have big areas of hair they really want to get rid of. For the men who want to do their own rough facial hair be prepard to get patches within the following months and many people saying man what is wrong with your beard.
I switched specialists for the reason that after a few years of Ipl, I figured the benefits weren't worth it. Then I bought the laser hair removal system. They will warranty that after two years’ time in case that hair grows back again, all the treatments are actually absolutely free. They've got both lazer and also Intense pulsed light, but I want to stay with laserlight now as Ipl did not seem to decrease the quantity of hair returning.
I was doing tria laser hair removal system applications for at least a year now on my mustache. Before then I had a very thick dark beard. I'm proud of the results. The actual hairs are very thin and I haven't gone back for treatment for approximately a few months. For certain i will point out that it did hurt just a bit on my upper lip and there were times that I had a little redness on my chin. I recommend Ipl to those with heavy hairs or have large spots of hair they really want to eliminate. For the men that might like to do their tough facial beard be prepard to get spots over the following several weeks and many people saying dude what is up with your facial hair.