a couple questions regarding fluid flow

[ChemEng84]

i'd like to have an indepth idea about these topics;
1) what is Choke velocity and how to calculate it? is it just mach 1?
2) what's surge in compressors and minimum flow in pumps? and how to calculate them?
3) regarding compressors we have 6 turbocompressors that are controlled via a system named CCC, where the speed of the compressor is controlled according to the suction pressure ( pressure upstream the compressor) i'm told that when the pressure upstream compressor increases, the speed of the compressor decreases and vice versa. could anyone explain to me howcome this happens, as far as i understand if the preassure upstream the comressor increases its supposed to increase the spped of the compressor to decrease the pressure again not to decrease the speed!!!!

 

[kchan711]

Answering #1 with gas as the fluid, the choke velocity is the speed of sound. This is dependent on the type of gas (molecular weight), temperature among other things.

For a perfect gas:

a=(kRT)^(1/2)
k= cp/cv (specific heats)
R= lamda/molar mass gas = 8314 [J/(kmol-K)]/ MMgas [g]
T= temperature in Kelvin


Kyle

Kyle Chandler

http://www.chiefengineering.net

"To the Pessimist, the glass is half-empty. To the Optimist, the glass is half-full. To the Engineer, the glass is twice as large as it needs to be!"

 

[kchan711]

a = speed of sound

Kyle Chandler

http://www.chiefengineering.net

"To the Pessimist, the glass is half-empty. To the Optimist, the glass is half-full. To the Engineer, the glass is twice as large as it needs to be!"

 

[JLSeagull]

See the curves for centrifugal pumps. Centrifugal pumps are simple compared to centrifugal compressors.

I think of surge in centrifugal a compressor as stalling the blades. Others can provide a more elegant and technically accurate description. Many articles and even books are published on this topic. Many variables exist. Some compressors are constant speed; others variable speed. Some such as air compressors have a constant fluid density. Some process compressor densities can vary greatly. Compressor Controls Corporation is a control systems company that specializes in surge control and load sharing. Read literature specific to the CCC scheme for your installation. PetroTech and others provide compressor controls.

Generally, the surge controls estimate the minimum flow required to avoid surge anywhere along the surge line. Review the compressor capacity charts for each machine of interest. The surge controller simulates the compressor curves and control to a line shifted away from the surge points. As the flow rate is reduced toward surge the controller senses a rapid loss of flow or pressure change and immediately opens the surge recycle valve (perhaps vent for air).

The capacity controls may increment the speed (or inlet guide vanes) for all machines base load some machines and control the speed of others. Load sharing can use suction or discharge pressure, mass flow rate, differential temperature (perhaps dp) etc. associated with the efficiency for each machine. (It is best that all parallel machines were identical when shipped). Load sharing attempts to operate at the maximum efficiency for the compressor combination while avoiding surge.

 

[sailoday28]

Choke velocity depends upon the process. For a perfect gas, constant specific heats
Adiabatic flow, see response of kchan711
Isothermal flow, V=sqrt(RT)

 

[BigInch]

Minimum stable pump flow, where flow is not sufficient to carry away mechanically generated heat with the flow itself. A thermal analysis will determine what flow can be sustained, but is generally equal to 10-15% of BEP flow for centrifugals.

"Less than 1% of the energy moving a car goes towards the driver."
Amory Lovins - The Oil End Game

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[ChemEng84]

is there a choke velocity for liquids? and how can it be calculated?

 

[quark]

No

 

[25362]

Instrument engineers refer to the limiting vaporization of liquids crossing a restriction such as a valve as choked flow.

Les Driskell in his book titled Control-valve selection and sizing published by the ISA (para 8-4) when referring to cavitation and its effects on valve capacity, says:

...Observe that the effect of vaporization is to reduce pressure recovery. Once the vaporization at the valve is fully developed, the pressure at the vena contracta reaches its lower limit. If the downstream pressure is then further decreased, the result will be more vaporization, but the vena-contracta pressure is not affected and the flow rate stays the same. This is called choked flow...

 

[BigInch]

Yes also at the velocity of sound in a liquid, but you are more likely to experience choking across a valve by flashing, when downstream pressure reaches the fluid's vapor pressure.

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

**********************
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