sand filter clogging instrument

Charbel · Feb 10, 2017

dear,

I was searching the net in order to find how to measure filter clogging in sand filters, I found something from degremont that, when a pre-set degree of clogging is reached, filter washing must take place.
But how clogging is measured? I found in other places that they are using PDT for measuring clogging, but I could not understand how this sensor works.

I really appreciate any feedback on this issue,

thank you,

Charbel

Bit_Bucket_07 · Feb 10, 2017

Filter clogging is typically detected by measuring the differential pressure between the inlet and outlet side of the filter. As clogging increases resistance to flow, the pressure drop across the filter will increase.

BobB · Feb 10, 2017

Yes - differential - I do swimming pool projects.

Charbel · Feb 10, 2017

dears,

thank you for your replies,

actually as stated below this a sand filter V type, if we want to install differential pressure sensor on each sand filter, one leg of the DPS will be on the downstream pipe coming out of the sand filter, where do you think the other leg will be located in?

your feedback is really appreciated,

charbel

Bit_Bucket_07 · Feb 10, 2017

Charbel said:
dears,

thank you for your replies,

actually as stated below this a sand filter V type, if we want to install differential pressure sensor on each sand filter, one leg of the DPS will be on the downstream pipe coming out of the sand filter, where do you think the other leg will be located in?

your feedback is really appreciated,

charbel

If you're talking about a non-pressurized, gravity fed, open sand bed filter, then one would need to infer clogging by monitoring reduction of the output flow rate through the filter, rather than by measuring pressure drop.

Charbel · Feb 10, 2017

Bit_Bucket_07 said:
If you're talking about a non-pressurized, gravity fed, open sand bed filter, then one would need to infer clogging by monitoring reduction of the output flow rate through the filter, rather than by measuring pressure drop.

dear,

thank you, from my understanding, pressure also give an indication of flow, low flow will lead to low pressure. please advice.

Tom Jenkins · Feb 10, 2017

Sand filter clogging is typically measured in one of two ways. The most common is to simply monitor the water level in the filter. As the filter clogs the head loss increases, and at some point the rising water level trips the backwash cycle. The flow rate through the filter may or may not be constant, since water usage varies during the day. The filter manufacturers don't worry about that - they just want to backwash before the water runs over the filter wall.

In less common arrangements the head on the filter bed is maintained constant by throttling at the discharge. In those cases you could trip backwash by monitoring flow. As the filter clogs the flow will drop for a given head.

In almost all systems the filter is also backwashed on a time basis, whether it needs it or not.

I suggest you get a copy of Metcalf and Eddy - it is the standard reference for wastewater treatment. That appears to be your field.

Charbel · Feb 10, 2017

Tom Jenkins said:
Sand filter clogging is typically measured in one of two ways. The most common is to simply monitor the water level in the filter. As the filter clogs the head loss increases, and at some point the rising water level trips the backwash cycle. The flow rate through the filter may or may not be constant, since water usage varies during the day. The filter manufacturers don't worry about that - they just want to backwash before the water runs over the filter wall.

In less common arrangements the head on the filter bed is maintained constant by throttling at the discharge. In those cases you could trip backwash by monitoring flow. As the filter clogs the flow will drop for a given head.

In almost all systems the filter is also backwashed on a time basis, whether it needs it or not.

I suggest you get a copy of Metcalf and Eddy - it is the standard reference for wastewater treatment. That appears to be your field.

dear tom,

thank you for your usual assistance, I have check the Metcalf and eddy related to wastewater treatment, they indicated that the headloss across the filter can be measured as included in the attachment.
how do you think the headloss is measured?

thank you,

Charbel

Charbel · Feb 10, 2017

Charbel said:
dear tom,

thank you for your usual assistance, I have check the Metcalf and eddy related to wastewater treatment, they indicated that the headloss across the filter can be measured as included in the attachment.
how do you think the headloss is measured?

thank you,

Charbel

please refer to as-built from contractor which are showing PDT and they are naming it as filter clogging device, that is why I am trying to figure out where this sensor will be located and if it can be located.

Tom Jenkins · Feb 10, 2017

The level transmitter (LT) will provide the headloss, since the level of the underdrains and the top of the filter bed are fixed.

I have no idea what they do with the differential pressure transmitter (PDT) or where they would connect the two transmitter ports. You indicate that this is an as-built drawing, so I suggest a plant visit or contact the operators for information. They may have one port in the filter above the top of the filter bed and the other open to atmosphere. In that case the PDT can be used as a level transmitter. Then the question becomes what is the purpose of the LT?

labeledas · Feb 10, 2017

They might be controlling at a fixed level with the LT and use the DP for head loss of filters. I seem to recall one installation I worked on had it spec'ed like that as well, they had a reason can't remember it though so not sure if it was a good one, or and we just want to try it like this.

Tom Jenkins · Feb 10, 2017

labeledas said:
They might be controlling at a fixed level with the LT and use the DP for head loss of filters. I seem to recall one installation I worked on had it spec'ed like that as well, they had a reason can't remember it though so not sure if it was a good one, or and we just want to try it like this.

Interesting concept, but I can't believe it worked like they expected. By definition, if you maintain constant level you have to do it by decreasing the flow as the media plugs. The head loss in an open filter, by definition, is equal to the water level.

labeledas · Feb 10, 2017

I don't know it ever worked as per engineer specs, I was puzzled how it would work as well. I was only the commissioning monkey terminating wires and loop checking. the client never used an automated backwash feature anyway the always and wanted to continue to do it manually based on time.

Saffa · Feb 10, 2017

With gravity filters we monitor water level above the bed and control the position of the discharge valve in order to maintain a constant level above the bed which results in a steady flow through the filter.

As the bed binds up the outlet valve opens progressively up to 100%. We either trigger backwash on valve open at 100% for more than 15 minutes, or level above the bed reaching a high setpoint. Works pretty well.

RET · Feb 10, 2017

Saffa said:
With gravity filters we monitor water level above the bed and control the position of the discharge valve in order to maintain a constant level above the bed which results in a steady flow through the filter.

As the bed binds up the outlet valve opens progressively up to 100%. We either trigger backwash on valve open at 100% for more than 15 minutes, or level above the bed reaching a high setpoint. Works pretty well.

That makes complete sense to me.

sand filter clogging instrument

Charbel

Member

Bit_Bucket_07

Member

BobB

Lifetime Supporting Member

Charbel

Member

Bit_Bucket_07

Member

Charbel

Member

Tom Jenkins

Lifetime Supporting Member

Charbel

Member

Charbel

Member

Tom Jenkins

Lifetime Supporting Member

labeledas

Member

Tom Jenkins

Lifetime Supporting Member

labeledas

Member

Saffa

Member

RET

Member

Similar Topics