Choke tubes V. restriction orifices

[jonesey]

Can anyone advise as to the advantages7disadvantages of using choke tubes against restriction orifices on pump warm up circuits ? How does the choke tube actually work ? My company has, over many projects, used both methods, but no one can seem to give an adequate explanation as to when each type is used.

 

[Leclerc]

for restrictors/ against choke tubes:
can be installed easily between existing flanges.
can be changed relatively easily if wrong size.

for choke tubes/ against rstrictors:
larger bore, therefore less likely to blocked with lumps.

 

[davefitz]

Choke tubes are sometimes called capillary tubes. These are immune from the damage caused by cavitation or erosion/corrosion, if fabricated of 1% Cr alloy or stainless tubing. Since larger openings are used( compared to a single orifice) they do not plug as easily and their pressure drop characteristic does not change over time.

They work by the process of frictional choking of the flow, and the friction is distributed over the length of the tube. We have used them often in supercritical steam generators as bypasses around the startup control valves, typically with pressure drops of over 2700 psid ( 186 bar) and fluid enthalpies in the region of hot water thru wet steam.

On a simple level, the pressure drop is calculated backwards ( from outlet to inlet), in the case of possible flashing of the fluid to 2 -phase. If the enthalpy and outlet pressure is known from process data, one breaks teh pipe into 100 smaller subelements , assumes a flowrate, and back calculates what the inlet pressure, steam by weight, velocity is for each sequential element, working backwards.