What is Suction Pressure?

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Thread Starter

chillwin

Could someone please explain what suction pressure of a compressor is and how is would affect the flow going into it?
 
chillwin,

Suction pressure; kind of sounds like a contradiction, doesn't it?

But, it's the pressure on the suction of a pump or a compressor. And, some pumps and compressors require a positive pressure on their suction--not all pumps or compressor can draw air/gas/fluid into their suction. Net Positive Suction Head (NPSH) is a commonly used term to describe the required pressure on a pump's suction.

Now, without some positive pressure on the suction of many pumps and compressors the flow through the pump or compressor will be limited.
Compressors (centrifugal compressors, mostly) operate best (highest flow range) with a positive pressure on their suction. And, in some cases compressors have minimum flow requirements--for cooling of the compressor internals. Deadheading OR reduced flows can cause overheating of the compressor (or even centrifugal pumps).

And, for many large centrifugal compressors (especially on gas pipelines) they are designed to operate with a high pressure on the suction, and slightly higher pressure on the discharge--but it's really flow that they're trying to maintain. They can't "suck" gas into their suction if there's not sufficient flow (pressure) on the suction.

So, low pressure at the compressor suction is indicative of a possible low flow through the compressor, and again, design considerations must be maintained during operation and many compressors are designed to operate with a minimum pressure at the suction in order to maintain the minimum rated flow, many times for cooling or to prevent over-heating.

Does this help?
 
It's perhaps easier to think of a pump first.

A pump is sitting beside a tank that's full to the 10 foot mark. The suction pressure is equal to the pressure from the surface to the centerline of the pump suction. in this case 10 feet of suction pressure.

If the tank was on the 1st floor and the pump in the basement 12 feet below the suction pressure is 10 + 12 = 22 feet.

In the case of a compressor open to atmosphere the suction pressure is zero psi. If it has a inlet filter, the pressure would be a little below zero.

Gasses are more complicated because the mass is effected by the pressure.

Hopefully a compressor guy will be along shortly.
 
Thanks for all the replies. So what exactly determines the suction pressure and what causes it? If for example a gas compressor has 2 bar suction pressure does this mean that the pressure of the gas flowing into it have to be greater than 2 bar? How does this affect the flow rate of the gas at the suction and discharge points of the compressor?
 
Dear chillwin;

> what exactly determines the suction pressure and what causes it?

If the talk is about gas compressors used in Oil & Gas Field i will say upstream process. And if your question is about compressor design I will say Compression ratio and Thermodynamics laws.

Centrifugal Compressor is a <b>dynamic</b> and continuous flow machine. The rapidly rotating component (Rotor/blade) accelerate the gas as it passes through the blades converting the velocity to pressure (partially in the rotating blades and partially in the diffuser). So gas pressure must rise between the inlet (section) and the compressor discharge.

Centrifugal compressor is also a <b>polytropic machine</b> governed by few thermodynamics laws:

P1V1^n = P2V2^n or T2/T1 = P2/P1)^((n-1)/n) where n = polytropic exponent

Regarding the second formula and usually, discharge temperature (T2) will limit the compression ratio (mechanical design limits)

Regards
Karim
 
chillwin,

Roy Matson's excellent response very accurately describes what determines suction pressure--it's determined by the pressure at the intake of the pump or compressor (the suction). In the case of the tank, the total distance from the pump inlet of the top of the liquid in the tank determines the pressure at the inlet (suction) of the pump.

In the case of a compressor, the pressure in the line connected to the inlet (suction) of the compressor determines the pressure at the suction of the compressor. If it's coming from a pressure vessel with 2 bar of pressure, the pressure at the suction of the compressor will be 2 bar (minus any piping losses).

If a compressor nameplate requires 2 bar at the suction, that means it will make rated flow when the pressure at the suction is 2 bar. If the pressure is less than 2 bar, it won't make rated flow and pressure at the discharge. (I think the flow into the compressor has to equal the flow out of the compressor, unless there's a leak or the compressor can manufacture more of the gas that it's flowing.)

Again, some pumps/compressors can't create a vacuum (what many would call a "suction") at the inlet to the pump/compressor (also called the pump/compressor "suction"). In order for these pumps/compressors to flow fluid/gas they have to have a positive pressure at the inlet (suction). And to flow rated throughput at rated pressure these pumps/compressors need to have a positive pressure at the inlet (suction).

My explanation is probably very simple, and may not be applicable to all compressors. But, the centrifugal natural gas compressors I have worked on operated like this. They all had to have some positive pressure at the suction (inlet) in order to raise the pressure and flow rated gas. If the suction pressure was too low, the compressor would not flow rated gas, and in some cases would trip off line (the compressor suction would be closed and the compressor prime mover would be stopped).
 
chillwin,

I want to add one more thing--most centrifugal gas compressors can't draw a "suction" (vacuum) on their intake (suction). (Isn't the English language wonderful?) And they have to have some pressure, which ensures the flow of gas into the compressor to make rated flow. Without sufficient pressure "behind" the gas, there won't be sufficient flow into the compressor, which means there won't be rated flow through the compressor.

It's kind of like amperes--it requires volts (the "pressure") behind the amperes to make amperes flow in a circuit. Amperes are what makes things happen, but it's voltage that provides the "pressure" to make the amperes flow. They are both necessary for flow and for rated flow. (It's not a great analogy, but it's the best one I can think of this evening.)
 
If you look at the data sheet for a pump or compressor there will be a minimum suction pressure listed. This is the minimum inlet pressure required for the equipment to operate at it's rated flow or pressure increase; and for a pump not to cavitate.
 
chillwin,

I want to add one more thing--most centrifugal gas compressors can't draw a "suction" (vacuum) on their intake (suction). (Isn't the English language wonderful?) And they have to have some pressure, which ensures the flow of gas into the compressor to make rated flow. Without sufficient pressure "behind" the gas, there won't be sufficient flow into the compressor, which means there won't be rated flow through the compressor.

It's kind of like amperes--it requires volts (the "pressure") behind the amperes to make amperes flow in a circuit. Amperes are what makes things happen, but it's voltage that provides the "pressure" to make the amperes flow. They are both necessary for flow and for rated flow. (It's not a great analogy, but it's the best one I can think of this evening.)
I am a new member here and had a small doubt after reading the posts. what about the the pressure difference between the compressor discharge and compressor inlet once the compressor is on? Suppose the compressor is rated at 50bar inlet pressure and when the inlet pressure is on decreasing trend any suction pressure will be be created due to the differential pressure, I think so, and I guess there would be a critical inlet pressure limit beyond which the compressor cannot run, based on the instrumentation settings it would flash low inlet pressure signal and the rotational speed will be reduced inline with the reduction in the inlet pressure beyond the critical limit.
 
Krishnaa26,

Good thoughts, all. But, all compressor control systems are not programmed the same (to do the same things, or even what we think should be happening).

Best to consult the graphs and documentation provided by the compressor manufacturer(s) for the most important information and details--specific to the machine in use at the site where you are exchanging your work for money. That's the only thing that counts: What the machine's manufacturer specifications and operating guidelines and limits say. Next, if there appear to be problems with how the machine is actually operating, well, then the compressor control system (or the plant control system) needs to be analyzed to see if it is programmed and configured according to the machine's manufacturer's specifications and guidelines. Many times small changes can have very beneficial effects, and many times the programming and configuration is not exactly what the manufacturer specified or desired; quite often programming and configuration is based on the programmer's prior experience with the same or similar machines. And, compressors, like many steam turbines in prior years, are usually one-of-a-kind machines, designed for a particular application or location and can have small, but important, differences.

To my way of thinking, if the pressure on the suction side of a compressor is, say, 20 barg, and it is decreasing "slowly" then it's not very likely that the compressor is going to start "sucking" harder--there's almost 20 barg on the suction (decreasing). But, when the pressure decreases to some point, there is usually an alarm to alert an unconscious operator to the situation, and who will hopefully initiate the proper response to restore the suction pressure or at least keep it from falling below a level which will cause the machine to trip (emergency shutdown) to protect itself.

But, again, that all depends on the control system and how it's configured and programmed for the machine at the particular site. And, again, they're not all configured and programmed the same, or even similarly (though many are).
 
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