Sizing of the actuator is critical for long-term performance. I typically specify that all actuators be sized for one-and-a-half to two times the maximum torque required of the valve. You can save a little money by going with smaller actuators (and many suppliers will do this to make their bid more competitive), but about a year later when things start gumming up, you will have problematic valves and the folks in the maintenance department will hate you. I also tend to favor higher wattage solenoids for the same reason. If you have to use low-power or intrinsically safe solenoids, you need to make sure you have exceptional air quality.
Greg: Do you have any parting words on instrument upgrade projects?
Hunter: I have three pieces of advice for anyone beginning such a project.
- You can cut corners in a lot of places on a project, but buying cheap instrumentation is not the place to do it. You get what you pay for. The equipment may work fine through start-up, but over the course of the next few years, the company will lose those savings many times over in maintenance costs and lost production.
- Take the time to understand your process and understand the instrument options at your disposal. Do not toss the specs to your supplier and let him fill in the blanks. His priorities may be dramatically different from your own, and he almost certainly will not understand the specific requirements of your particular situation.
- Proper instrumentation design does not end with the specifications. A perfectly specified instrument will perform miserably if it is not installed correctly.
Top 10 Things You Don't Want to Hear on Start-Up
10. "We just need the owner to be a little more patient." (Supplier expert, actual quote)
9. Don't bother with a checkout—just light it up! What is the worst that can happen?
8. We didn't do any simulation or testing. We decided that would spoil the adventure.
7. I don't understand. It fit fine on the drawing.
6. "Cool, this is my first time in a real plant!" (supplier expert)
5. I tried to open the valve and nothing happened. Wait! The same valve on the other reactor just opened!!
4. Should the VFD drive smoke like that?
3. I don't understand. I am sure I left all your tools and radios in a box right here.
2. The CEO is on the phone holding for you.
1. Boom!! What was that?!?!
Ten more things you don't want to hear on an instrumentation project start-up
20. We standardized on the gain and reset setting of 1 used during checkout with tieback filters. (Actual case)
19. We set the controller gains equal to the proportional bands in the old controllers. (Actual case)
18. You mean you didn't label the wires before you lifted them?!?!?!
17. The civil, structural and mechanical schedules have all slipped a week each, but the start-up deadline has not changed; you have 4 ½ hours to do your install/checkout.
16. We let IT rebuild all your control computers to meet their standards. That's ok, right?
15. Yes it is serial number 1, but it worked great in the lab!
14. We've never seen this problem before.
13. The thermocouple wiring is completely finished. Red is positive, right?
12. This is "Sleepy" Murdock. He's been assigned to work with you as your I/E tech for check out.
11. The integration company has lost too much money on this start-up so they flew home last night. (Actual quote)