rate of change limit on rapid gravity filters
rate of change limit on rapid gravity filters
(OP)
We have an instruction from our client that the rate of change of flow through their rapid gravity filters should be limited to 5% per minute. I am trying to write the control system for a new RGF plant at the moment and I need to know if this is 5% of the flow setpoint we are aiming for, or 5% of the actual flow through the filter. As is always the way, there is no one around at the moment to take my call and clarify their instruction. I know other clients have requested similar rate of change limits in the past so I am wondering how common this is, why it is done and if anyone can advise which flow rate I should be looking at in the % calculation. Obviously I'll keep trying to find out from my client what he wants as well, but your help would be appreciated.





RE: rate of change limit on rapid gravity filters
It is desireable to maintain a constant rate or, when operation is intermitent, accelerate or deccelerate the rate of flow very slowly while bringing a filter online or taking one offline.
In this case, it sounds to me like your client is asking for the rate of flow change to be limited to 5% per minute when ramping up or down. This is a reasonable request. I wouldn't say it is common practice, but it certainly is a good design practice.
On the otherhand, the request could be related to the "tuning" of the rate of flow control device. The client might be using the specification to attempt to "dampen" the rate of flow controller so that it will not occulate too widely when trying to reach the design set point.
In either case, both issues should be addressed with the goal of minimizing abrupt changes in the rate of flow through the filter.
S. Bush
www.water-eg.com
RE: rate of change limit on rapid gravity filters
From "Water Quality and Treatment" published by AWWA.
It is probably referring to making the change from existing flow conditions to revised flow conditions over a 20 minute period (20 X 5% = 100%). (Revised Flow - Existing Flow)/20.
RE: rate of change limit on rapid gravity filters
Thanks for your help.
RE: rate of change limit on rapid gravity filters
preferrably the particle count level is low enough to allow you to put the filter back into service safely. We found that a particle count of 75 or less worked okay. This practise has been used now for a few years with good success, provided the filter was washed properly to start with.
We also noticed that when the ramp rates are varied above a certain level,that is when the filter flow rates are varied usually up, we do get a minor rise in the turbidity levels. These did not produce any spikes.