I have an application where during a start-up operation a pressure control valve is required to open slowly and then after a time resume normal control. This is a pneumatically operated valve and my client insists that the speed should be controlled mechanically by restricting air to valve actuator. How can I tune a valve that has dramatically different speeds of response for different directions of travel?

 

I have a crude idea and maybe someone else or you can refine it. Air signal from I/P goes to the actuator via two parallel paths. One through a restricter and one through a change over relay or solenoid valve directly to the actuator. When the machine is starting up the solenoid valve should be unlatched and the signal goes through restricter. Once the machine is running normal, solenoid can be latched manually or autmatically and the signal goes through solenoid valve. A pneumatic change over relay can be used instead of solenoid valve.

 

In this business the adage, ‘the customer is always right’, isn’t always right. You need to build a case to convince him to allow you to accomplish the desired valve (and process) behavior in software; aside from the desired startup behavior, you can
1) build in tunability (adjust directional speed of response) without having to diddle with different orifice sizes.
2) Remove the restrictive behavior once the transient startup condition has elapsed, thereby improving overall loop performance.

His requirement should be considered a functional requirement (actuator speed must be slowed during…because…’) rather than a design requirement (‘actuator speed must be slowed via use of restrictive orifice…’). Make sure too you know all his reasoning for doing this; try to be nonconfrontational… that you’re just seeking to understand this special requirement.

Offer to develop a prototype with his input and demo it to him. Some clients like to leave their mark on design; by offering to develop a strategy with his input, he gets that.

Good Luck

 

Is your PID control from a PLC. If it's from a PLC why can't you have two seperate PID loops. One loop will be configured to have the slow response and the other will be for normal operation. Or why can't you just have some programmed rungs to open the valve incrementley over a set time period and once the valve is open to a predetermined percent have the PID loop take over.

Regards,
Malachy

 

Suggestion:

Is the valve controlled by a PLC or PC? If so, you can have the PLC/PC step through opening the valve slowly over the period of time that it's supposed to open slowly. After that period of time, you could have it controlled as you normally would.

 

Hai,

What I can suggest is that you can two PID loop one for Start-up & other for Normal operation based on the condition start-up & normal operation you can switch over PID loops.

With best regards,

S. Elavazhagan,
Engineer - I&C,
TCE Consulting Engineers Ltd.,
Bangalore, India.

 

I would rather do it in software using a smart valve positioner like Smar FY301.

However, I guess it can be done mechanically by putting some restrictions on the input and vent.

Jonas Berge
==================
[email protected]
www.smar.com

 

try exhaust speed controls or a 3-15 ma positioner of the like
[email protected]