Drain pipe 1

[semarang]

Hi all, I've got a problem in drain pipe that goes to"dance". In our desal plant, there is a brine pump in brine heate who pumping the condensate that formed. The discharge pressure of the pump is varies from 3 - 4.5 bar. Normally its operation condition is 4.5 bar. These pressure variation make a "punch" to the elbow of the discharge piping. I really want to know what is the source of the pressure variation? or maybe there's another source of the "dancing" pipe? (the pipe size is 40 mm, pump: ebara 2910 rpm, 117 l/min, 40m, 5.5 kW)

Dwi Handoyo S

 

[PUMPDESIGNER]

I think I understand your problem, you are obviously struggling hard with our wierd language.

The best way to look at the problem, if I understand correctly, is that you are seeing changes in fluid velocity. Each time the velocity changes rapidly, the pipe "dances".

Now, if that be true then you are looking for anything that would change the velocity rapidly. Some items, based on my casual experience are:

INTAKE CONSIDERATIONS:
1 - Air or other gasses entering the pump, from leaks in the intake line, from the end of the suction picking up air from a vortex or swirl, or from depressurization of the liquid as it travels through the intake line if the liquid is super saturated or highly saturated with gasses that can come out of solution due to the drop in liquid pressure inside the intake line or pump internally.

2 - It is possible that the pump or it's intake structure and supply is improperly applied or selected, resulting in internal pump misbehavior such as cavitation. However those things DO not usually affect the discharge pipe systems violently unless very extreme. I will not go into this area at this time unless I know that you can understand and do calculations for NPSHr vs. NPSHa, etc.

DISCHARGE ITEMS
1 - Rapid velocity changes due to rapidly closing or opening valves, especially relief valves, fast manual closure of ball or butterfly valves, malfunctioning control valves, chattering valves.

2 - If an accumulator or pressure tank is present then that item may have problems such as incorrect settings, no air pre-charge (water logged), etc. There must usually be some form of energy storage or static head on a discharge line to prevent rapid changes in pressure.



PUMPDESIGNER

 

[semarang]

Thank's 4 the reply Mr. Pump Designer.

I believe that the phenomenon is come from the malfunctioning control valve. We use CRV with positioner. The CRV's positioner is to wide to close, the closing stroke is too long. So, there is a moment with no flow at all (in an extreme condition). Is there any possibility of any phase change, from water into the vapor? because the discharge line is connected with the hotwell condenser with its low pressure.

Thank you very much.

Dwi Handoyo S

 

[PUMPDESIGNER]

Phase changes in your fluid occur very easily.
But phase changes must be extremely large and extremely well developed to cause sudden velocity changes.
Phase changes result in vapor bubbles of gas, which are better thought of as cavities.
In borderline conditions (typical) the vaporization of the liquid increases head which prevents further vaporization.

For phase changes to cause large velocity changes the tiny cavities of vaporized fluid must become so numerous and so dense that they coalesce into larger cavities called super cavities. And that ain't so easy to do.

Cavities form on the discharge side of the pump where severe turbulence occurs:

Any control valve where the disc is riding close to the seat.

Where the pressure drop across the seat is very high due to high flow rates.

Any relief control valve venting to a very low pressure area such as atmosphere.

Also, extremely fast velocity changes occur in discharge lines easily when there is no accumulator or static head and valves attempt to operate in high flow situations.

PUMPDESIGNER