Hello,
My plant uses nitrogen for plant air incase our air compressor trips off and loses normal plant air.
Normally the nitrogen air is regulated with a regulator at 200 psi on upstream and 100 psi downstream.
When plant air is loss, we are seeing nitrogen at 66 psi on downstream of regulator and about 194 psi on the upstream.
So even though regulator holds 100 psi when blocked in and everything is normal, when nitrogen air is needed for back up, we see 66 psi on downstream on regulator.
Any help with this issue please.
DCS Newbie,
Based on what you are describing it sounds like you are experiencing what is known as droop. A pressure regulator is a mechanical device and as the downstream flow demand increases, in order to get more flow through the pressure regulator, the downstream pressure must drop (or droop). The higher the flow demand the higher the droop. I have attached an example graph below.
When selecting a pressure regulator you will want to determine your maximum flow rate and then--using the manufacturers bulletin (or data sheet)--determine how much droop will occur. In your situation is looks like you are experiencing about 34% droop at your normal flow rate. All pressure regulators will droop but the amount of droop can be reduced by using either using a different model of regulator or a different type (i.e. pilot operated). Pilot operated regulators will have less droop than a self-operated regulator.
More than likely you will need to evaluate installing a new pressure regulator. One that will give you less droop at your required flow rate.
Michael Calaway
michael.calaway@emerson.com
In reply to Michael Calaway:
Michael Calaway said: DCS Newbie, Based on what you are describing it sounds like you are experiencing what is known as droop. A pressure regulator is a mechanical device and as the downstream flow demand increases, in order to get more flow through the pressure regulator, the downstream pressure must drop (or droop). The higher the flow demand the higher the droop. I have attached an example graph below.
Thank you Michael for the detail response.
I am still trying to understand the droop scenario.
So how does the pressure drop from 100 psi to 66 psi? I am not understanding. Are you saying that the regulator can not meet the flow demand and its dropping pressure too much trying to reach normal flow demands?
Thanks for clarification.
In reply to DCS Newbie:
Yes, that is correct. The regulator by design is trying to match the downstream flow demand and maintain a constant downstream pressure. As the flow demand increase, in order to get more flow through the regulator, the pressure under the diaphragm must drop opening the regulator more.
As I mentioned above different types of regulators will droop differently (i.e. pilot versus self operated). However, one common reason for too much droop is incorrectly sized regulators. The regulator may be too small or designed for a higher of an inlet pressure.
Here is a depiction of the parts of a regulator that might help you visualize what is happening inside the regulator. Note that the picture below is a self operated regulator and if you have a pilot operated regulator it will work differently but the same concepts apply.
Thank you Michael. Now I understand droop.
I believe my problem is drooping because the normal operation is the downstream flow is blocked off with an ON/OFF valve unless plant air is loss and nitrogen air is required til the air compressor turns back on. The regulator is used for nitrogen flow to the instrument for air back up.
As soon as as the on/off valve opens (nitrogen air needed), the pressure drops from 100 to 66 psi. Which means the flow is maintained, but with cost of major reduced downstream pressure.
Now, I will review the existing sizing data for the regulator. I believe the droop selected for this regulator is too much. I read somewhere that self regulator have about a 10-30% droop. So if my downstream pressure is 100 psi at no flow, then it should go to about 70% with flow needs.
Thanks,