Flow through Plan 11 1

[RS82]

Seal vendor has asked for the tubing to be 3/4 to minimize losses as the vapour pressure margin is low and we will need close to full discharge in the seal chamber..but our connection on the casing is 1/2NPT..if we have a 1/2Px3/4T connector, does this not restrict the flow? Should we go with starting 1/2tubing?

 

[bimr]

The 1/2Px3/4T connector will have the lowest loss headloss. The headloss would be slightly higher if you continued the 1/2-inch size and then transitioned with a 1/2x3/4 reducer.

Having said that, the headloss will be so slight that it will likely not make that much difference.

 

[1gibson]

If you only have a plan 11, where is the flow going? How can you worry about restriction, or losses, with no flow?

Discharge pressure below the seal chamber throttle bushing, discharge pressure in the stuffing box due to plan 11 connection from discharge.

 

[Pumpsonly]

There should not be a concern of pressure losses in Plan 11 piping. Most of the cases, a RO is needed to restrict the flow rate and pressure to the seal chamber. If the vapor pressure margin is low on the discharge pressure, you will surely have problem on the suction side.

 

[GaTechTheron]

RS82,

Technically, I would agree with the pump vender. With the same pressure difference between the start and end of a 1/2" or 3/4" tubing lines, the fluid velocity in the 1/2" line will always be higher, and thus exhibit a larger pressure drop.

However, we are talking about tubing and the differences are likely negligible.

If you have an issue with VP margin, you may want to:

1) Double check the clearances. I would consider closing the diametrical clearance of the throat bushing, and opening up the orifice size in the P11 line.

2) Verify the calcs you are using to determine seal chamber pressure as the calculation for flow through a circular annulus and an orifice in series is often misunderstood and over simplified.

 

[GaTechTheron]

Pumpsonly,

I generally agree with you.

However, consider the situation of a light hydrocarbon, and the vapor pressure margin as dictated by API682.

In this case, you could be fine on NPSH3, but have issues with VP margin in the seal chamber.