Low-Bleed or Standard Relays for Control Valves?

Happy New Year everyone! I hope your holidays helped to recharge your batteries and get you off to a great start for 2018.

I saw this exchange in my email over the holidays about the effects of using low-bleed relays with Fisher DVC6200 digital valve controllers for control applications as compared to standard relays. This question was in response to a blog post from many years ago, Positioner Pneumatic Pressure Bleed in Throttling Control Valves.

Emerson’s Riyaz Ali and I were copied on this email. Riyaz responded:

Emerson's Riyaz Ali

Generally, we recommend a low-bleed relay for use with on-off, safety shut down / blow down or those applications which remains static in one position, namely “Full Open” or “Full Close”. To bring the valve position to its full Close or Full Open position (travel stop), smart positioners provide full output pressure, which essentially equals supply pressure. Low-bleed relays are ideal for such low air consumption applications. Thanks to technology advancement that DVC has new feature called “pressure control mode” which does not allow output pressure saturation equal to supply pressure at travel stop, but still controls without reaching saturation.

In such cases, low bleed relay is ideal selection to minimize air consumption.

However, for a control throttling application, I would highly encourage the use of a standard relay to get better control performance. If an application such as APC (advanced process control) requires performance level of 1/8% span controller current change, standard bleed will be better option, although low bleed relays generally are good up to 1/4% controller span change.

The instrumentation manager at this refinery and petrochemical plant followed up with these questions:

Can you please explain on what happens to low bleed DVC6200 when the signal change is 1/8% (<1/4%)? Does the low bleed DVC6200 respond slowly with time lag or no response at all?

Can we get the exact value of controller change which can be handled by this low bleed relay and its time lag to respond if the signal change is 1/8%?

Riyaz responded back:

Generally for control valves, dynamic air consumption is a more appropriate term. Since vendors cannot provide dynamic air consumption data as it is variable. Hence, only static air consumption is published. Static air consumption may be good for an on-off device or valves which are in one unique position all the time.

Control valves are regulating devices and need to modulate quickly to meet newly computed controller setpoints to position the valve.

Standard pneumatic relays or low-bleed pneumatic relays on the DVC6200 work on a mechanical balance beam design principle. This keeps an aggressive active condition to take any changes to generate new output based on a new setpoint issued by the DVC algorithm based on controller demand.

As explained, both relay (standard and low bleed) will be responsive to 1/8% input span signal but due to the low volume of air, a low-bleed relay may be a little lethargic.

I wanted to free this shared expertise from the confines of email with the hopes that it might help you if you ever have similar questions.

You can connect and interact with other valve experts in the Valves and Valve Controllers & Positioners groups of the Emerson Exchange 365 community.

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