3 replies to this topic

[chem2013]

Dear All,

 

I am working on closed drain system of a gas processing plant.

We have a closed drain system  having a below grade closed drain vessel.

Drain lines connected to process equipment is either 2" or 3" and are for maintenance drain. These are above ground drain lines. Underground part of same drain line is indicated as 4". Slope is 1:100 for these lines.

 

Main underground drain header is provided with a slope of 1:400 and this is connected to closed drain drum.

 

We are calculating drain volume for each equipment with margin. Now the highest drain volume is considered for closed drain drum size.

 

Please advise on the followings:

1. Basis for closed drain drum sizing is correct or not.

2. How to determine draining rate for each equipment ?

3. How to verify process drain size 2" or 3" is correct or not ? Is it necessary to design for self venting for maintenance drain lines?

4. Do we need to perform underground header size for the maximum draining rate ? What % of liquid filing needs to be considered ? Is there any issue if main ug header has higher capacity than individual process drain lines?

5. Is it okay to have different slope for individual drain line (1:100) and main header(1:400) ?

6.If a process equipment is drained via level control into a closed drain header, will self venting criteria to be used for these process drain line sizing ?

 

Thanks.

Regards

 

 

[fallah]

chem2013,

 

1- In general, the highest remaining quantity of liquid corresponding to the volume of liquid below the low level switch after trasferring the liquid by process lines to other equipment, isolation and depressurization among all vessels/drums/columns....is the basis for closed drain drum sizing...

 

2- The draining rate is to be calculated considering the liquid in each equipment is drained under slight pressure by gravity...

 

3- The minimum line size of drain line is to be considered 3" to avoid blockage risk...i think considering self venting condition not to be necessary...

 

4- Yes, the header sizing should be performed bases on maximum draining rate. It might all drain lines to be connected individually to closed drain drum; then no need to header... 

 

5- Yes, the normal drain slope for lines of 18" and higher is 1:500; for lines of 6" to 18" is 1:250; and for lines of 3" to 6" is 1:80

 

6- Yes, drain from level instruments could be routed to closed drain system as operational drain...

[TS1979]

Dear All,

 

I am working on closed drain system of a gas processing plant.

We have a closed drain system  having a below grade closed drain vessel.

Drain lines connected to process equipment is either 2" or 3" and are for maintenance drain. These are above ground drain lines. Underground part of same drain line is indicated as 4". Slope is 1:100 for these lines.

 

Main underground drain header is provided with a slope of 1:400 and this is connected to closed drain drum.

 

We are calculating drain volume for each equipment with margin. Now the highest drain volume is considered for closed drain drum size.

 

Please advise on the followings:

1. Basis for closed drain drum sizing is correct or not. See comment on No.2.

2. How to determine draining rate for each equipment ? The draining rate should be determined by (1) how much liquid expected to be entrained in the process gas, (2) based on equipment pressure, temperature, how much gas will be liquidized in the equipment. 

3. How to verify process drain size 2" or 3" is correct or not ? Is it necessary to design for self venting for maintenance drain lines? Once you know the drain liquid flow rate, you will easily size the drain piping. Self venting is not necessary. However, a equilibrium line back to equipment may be required.

4. Do we need to perform underground header size for the maximum draining rate ? What % of liquid filing needs to be considered ? Is there any issue if main ug header has higher capacity than individual process drain lines?

5. Is it okay to have different slope for individual drain line (1:100) and main header(1:400) ?

6.If a process equipment is drained via level control into a closed drain header, will self venting criteria to be used for these process drain line sizing ?

 

Thanks.

Regards

[Sharma Varun]

Jus adding to what others have stated:

 

For gravity flow drain lines velocity is maintained in a range of 0.8 to 1.5 m/s, this is to allow self cleansing & avoid erosion/emulsion (oily water drains), so size the for 1 m/s velocity, if there is possibility of more solids you may choose to keep it 1.2 m/s.