2 replies to this topic

[nick_tri]

Hey all,
This is a follow up from my previous post on "multistage pump", where I was advised to use a positive displacement pump.
What I did was rework the material balance on the column bottoms so that my pump was getting a higher flowrate of recycle since the previous value of 0.0073m3/h was rediculously low. Now I have
Components: Acetone & Water
Temp: 54 deg C
Pressure in: 150 kPA and P out: 600 kPA
Flowrate: 0.21 m3/h
Total Head: 63.18 m
NPSHA: 16.29 m

Using Coulson vol6, Rotary gear pump with capacity of 0.05-500 m3/h and head range of 60-200m was determined to be the pump of choice. Now since Coulson didn't have design equations for rotary pumps I found some in Ludwig's Applied proces design. However I don't quite understand the equations well, maybe someone can clarify them for me.

1st Liquid Displacement Eqn
I need the theoretical displacement which I don't know what that is. Also Percentage entrained gas by volume ( I assume this is zero).

2nd Volume Displaced
n for RPM, since I assumed use of an External Gear Pump, I was goin to choose either 1750 rpm or 3450 rpm which are typical.
Theorethical displacement volume displaced per revolution of motor ???
Slip Value in GPM ??

How do i determine the efficiency of this pump ?

Also what other parameters do i need for design a pump like this in terms of size and mechanical design or design equations. Any help would be deeply appreciated.

Nick

[Art Montemayor]

Nick:

You are getting confused by only focusing on trying to find equations to do the thinking for you. You are also confusing all of us. You confuse us because you say you are using some equations. But, what are these equations? If you can’t type them out in the body of the post, then use Exel and Equation Editor – that is what Excel excels at. And when you cite a book, tell us the author, edition, & page. Ernie Ludwig no longer issues editions to his opus; now I believe that A. Kayode Coker has issued an updated version of Ludwig’s work. Which one do you mean?

Unless I am totally mistaken, no one in the University of the West Indies expects you to mechanically design a pump – much less a gear pump. You may be expected to specify the process conditions under which the pump must work successfully – and therefore dictate what conditions the pump must fulfill – flow rate, TDH, temperature, the materials of construction, and the driver. And I think that’s it.

Since you are unable (actually, incapable) of designing a pump, then you are at the mercy of those who do that for a living – the pump manufacturers. They issue their catalogs to allow you to select an appropriate model that will fit within your specifications. You have nothing to say about the availability of pumps with certain efficiencies. The capacities and the efficiencies are what they are. You either take them or leave them. While you can specify the speed of a pump (to suit your driver), the material of construction, the design flow rate, and the Total Developed Head, you have nothing to say or do about such factors as Slip, efficiencies, theoretical displacement, diameters, thicknesses, looks, aesthetics, and the price. You can sometimes pick your favorite color, but that’s about it. It’s just like enjoying your favorite music at Carnival: you can select any tune you like – as long as its what the band knows. You don’t have to know how to play the pans to enjoy the music; but if you want a certain tune ,you are at the mercy of those that can.

Have you been assigned to mechanically design this pump? If so, I doubt that we can be of much help here. I doubt that anyone here has ever designed and fabricated a pump. There are few engineers who have and those that do are employed in the pump manufacturing industry and they are not telling anyone how to design a pump simply because they will not help their competition and hinder their best interests. Please clarify this point so that we can be of some help – if we can.

[djack77494]

no one in the University of the West Indies expects you to mechanically design a pump

I see many questions of this type in this and other engineering forums. Even out in "the real world", unless you work for a pump manufacturer, you will not need to know how to design a pump. Unless you work for a compressor manufacturer, you will not need to know how to design a compressor. This concept can be extended to apply to most types of equipment, especially if they are complex. Even for something as simple as a storage tank, unless you work for a tank fabricator, you will not need to know how to design a tank. In many possible functions as a chemical engineer, you WILL need to know how to generate a process (or performance) specification for a pump, compressor, or even a tank. You use this document (hopefully supplemented with in person meetings and phone calls, etc.) to convey to those experts who do know how to design these types of specialized equipment just what your needs are. Then they can provide the equipment that meets those needs. While I haven't done a thorough job of reviewing the information you provided, I think you have very nearly completed all of the portion of the total effort that can be expected of you. (You might want to add something about materials of construction if you have preferences or know what is/is not suitable.)

Doug

P.S. I noticed you refering to some reference materials in several recent postings. That is a good practice, but you must keep in mind what those references are saying. If you have a certain flowrate and pressure and you go to a chart showing centrifugal pump types and suitability for flows and pressures, there are many assumptions built into that chart. Especially, it may not imply that you should use a centrifugal pump in that service at all.