1 reply to this topic

[deni]

I need a pump with suction design pressure 4 bar. unfortunately i dont have that kind of pump and only have a pump with 0.5 bar design. Can I use 0.5 bar suction design pump instead of 4 bar ?
What problem will occur ?

[Art Montemayor]

deni:

This is posted as a Student Problem, so I'll address it as such.

A real-life pump is rated on its pumping capacity (flowrate & head), its "frame" (maximum horsepower that can be imposed on it), and its ability to withstand internal process pressure (among other things such as corrosion resistance, max. temperature, etc.). You obviously are referring to your pump's pressure rating which refers to the maximum process pressure it can safely contain (in the new state, as judged by the manufacturer). You have not stated what type of pump you are talking about, so I'll have to assume the universally popular centrifugal type is in question.

Based on the above, the answer to your question is NO!

The obvious problem that will occur should you pressurize the pump casing above the cited maximum figure is that the pump could burst ("explode") and seriously injure, maim or kill near-by human operators and inflict mechanical damage to surrounding equipment. You don't don't want to be responsible for such an ignorant error nor do you want to be there when it happens.

Now, allow me to make an explanation to you as a student from my professional engineering point of experience:
Always -and I stress the word "always" - be intimately aware of the type of pump (or other related equipment) you are dealing with and the exact manner in which it mechanically operates. Just because you may be a Chemical Engineer doesn't absolve you from the engineering responsibility of knowing the characteristics and hazards involved in the particular type of equipment you are dealing with. For example, note that I specifically noted that you didn't mention the type of pump you are talking about. It just so happens that a centrifugal pump, while it may be operating with a 0.5 bar suction pressure and a 4 bar discharge pressure, still has to be capable of withstanding the maximum (the "dead-head") discharge pressure that particular pump can produce (which is normally always higher than the steady-state, maximum design pressure). Of course, I'm assuming that the process that the pump is attached to can't possible produce a higher pressure (if it can, then that pressure is the maximum).

What I'm trying to stress is that you must be aware of all the pressure possibilities that can occur when a plant is under actual operation - not just when it is under design as depicted in your calculations or on your simulator computer screen! The possiblity exists that the electric motor on your centrifugal pump could trip out due to an electrical over-load or power station failure and the pump's discharge pressure would instantly be subjected to the suction side of the pump! Therefore, it is customary to rate both the suction and discharge sides at the same maximum pressure. There is nothing to stop this from happening within the pump. The impeller would simply rotate in the opposite direction. A check valve is sometimes employed in the discharge line, but in the true sense of experienced safety, it can't be relied 100% to function correctly and in time, all the time. That's one reason why you have to bear in mind what I've tried to express above.

I hope this experience helps you out.

Art Montemayor
Spring, TX