rate of change limit on rapid gravity filters

[kchayfie]

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.

 

[sbush]

Granular media filters operate at their best when rate of flow is constant and sustained. When the rate of flow changes abruptly, premature breakthrough of solids can occur.

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

http://www.water-eg.com

 

[bimr]

"Filtration rate increases on dirty filters should be avoided or made gradually (over 10 minutes)."

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.

 

[kchayfie]

I have finally managed to speak to my client and he has confirmed that the 5% per minute limitation is to prevent breakthrough of solids, in particular when bringing a filter back on-line after a backwash. Apparently there is a spike of turbidity when the filter comes back on-line and this helps to minimise the impact of the spike.

Thanks for your help.

 

[5wp]

If you have the water, put the filter influent to waste until the turbidity level or
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.