Pressure Increase In a Bypass Line

[EGAtomic]

I am wondering how to correctly calculate the pressure increase in a bypass line. The situation I have is this: we have a 1"sched 40 bypass line (total volume of 434in^3) that is used to divert some process fluid so that it can be sampled for analysis. The 1" line has a tee with a 1/8" sampling line that is then connected to a pump, the pump extracts a set amount for analysis. After the pump takes its set sample it runs through a clean cycle and pumps solution back into the 1" bypass line so that the 1/8" line is clean for the next sample. The problem arises during the clean cycle: the bypass line is isolated from the main line by valves (so no flow in bypass line) and the pressure in the bypass line starts at around 10psi but when the pump goes through the clean cycle it rises the pressure inside the bypass line by about 30psi; and the pressure on the pump increases almost immediately increase to 85psi. The flow rate for the clean cycle is small only 2.5 liters/hr; so I can't seem to figure out why the pressure in the bypass line increases so much. I have done so many calculations already and I must be missing something because I only come up with a 2psi increase, so I am not sure where the 30psi is coming from. Any help would be appreciated

 

[zdas04]

You didn't say what your process fluid is. It matters. If it is water, then injecting a few million molecules into an isolated space would increase pressure enough to bust something. If it is dense phase CO2 then the real gas law applies and the pressure goes up reasonably slowly for increasing mass. Your pump is putting in 0.6% of the bypass volume every minute. With water I would expect pressure to increase over 300,000 psi per percent of volume added (i.e., 4.34 in^3 or 0.07 L). Immediately jumping to 85 psi seems really low to me.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

Post your calculation then we might be able to spot the issue but as zdas04 says doing this with liquid it really only takes a minute amount to increase pressure quite a lot.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EGAtomic]

zdas04 the volume is 434in^3 or 7.1L; the process fluid has a density of 1004kg/m^3 and a viscosity of around 800cSt so that is the fluid in the bypass line, the sample line (ID .085 with a length of 8")does have process fluid in it before it get flushed so that gives a pressure drop of around 33PSI but I can't figure out why the pump ends up at 85psi and the bypass inline increases by 30psi (from 10 to 40psi). When we do the clean cycle in the 1/8" line its with water. Also zdas04 if you can post your calculations or guide me to the correct equations that would be great; I will post my calculations as soon organize them from the scribble scratch.

 

[EGAtomic]

Sorry I should also say that the pump has a pressure valve that always puts 29psi of back pressure on the pump so that the internal valves on the pump close properly (its a diaphragm pump); so without any back pressure from the bypass line the pressure that the pump measures is 44psi. The pressure drop in the line is actually 15psi because the viscosity is 400cSt not 800cSt this was my mistake. This still doesn't explain the just to 85psi and an increase of 30psi in the bypass line.