4 replies to this topic

[shadow13]

Hi everyone,

I am currently doing the line list for our project. I am stucked up in the design pressure of the discharge line of air driven diaphragm pumps. We know that for centrifugal pumps, the lines and equipment downstream the pump should be designed for the shut off head of the centrifugal pump. In our company we use Shutoff = max suction P + diff'l P OR Shutoff = normal suction P + 1.25 (diff'l P) whichever is greater. So I set the design pressure of lines and downstream equipment (like tubes of heat exchangers) equal to the shut off head of the centrifugal pump. However, is this the same for air driven diaphragm pumps? I have heard someone that the shut off head for these kind of pumps is the supply pressure of the air that drives it? Is that correct? If so, why and how is that?

Your insights will be helpful.

Thanks in advance.

[djack77494]

shadow,
For an air driven diaphragm pump, the process side is essentially a chamber with inlet and outlet check or non-return valves. The flexible diaphragm allows the chamber's volume to change from a very small residual volume to the maximum available. The force that accomplishes these volume changes is the action of the fluid on the other side of the diaphragm - air. Thus, in the simplest configuration, you have your air supply on one side of the diaphragm and the process fluid on the other. At shutoff, the pressure of the two fluids would be equal.

Note that I stated "in the simplest configuration". That is because alternate configurations are possible. These can include configurations where a mechanical advantage is obtained for the driving fluid. This type of configuration would allow development of much higher shutoff pressures. However, the simpler configuration is also the most common.
Doug

[ankur2061]

shadow,

I found an interesting and informative post for air-operated double diaphragm pumps on the forum eng-tips. Here is the link:

http://www.eng-tips....d...551&page=10

Hope this helps.


Regards,
Ankur.

[shadow13]

Thanks Doug and Ankur.

I was just thinking, what if the pump discharge line suddenly gets blocked? will the diaphragm be ruptured if you don't have a relief valve downstream?

[shan]

Hi Shadow,

When the pump discharge line is blocked without a relief valve downstream, the pressure differential across the diaphragm will 0, because pressure values of both diaphragm sides are the same values (compressed air pressure value). Nothing will be rapture at 0 pressure differential even a piece of paper.

Regards

Shan