Stationary Inlet Guide Vanes and Inlet Nozzles in API 617 Centrifugal Compressors

[Rha257]

Dear All,

I have came across many customer specifications that limit the "Inlet Nozzle Velocity" to a specific value. Some specs call for a max value of 28 m/s, 30m/s, 46 m/s, etc. (Please see below article as well) I want to understand;

1. where these values are coming from?
2. why is there a limitation on the inlet nozzle velocities (other than a high-pressure drop across the nozzle)?
3. which nozzle velocity is being referred to (i.e. velocity at the inlet of the nozzle or velocity at the exit of the nozzle, since velocity at the exit of the nozzle is always greater than at the inlet of the nozzle). Also, I have seen one spec that states that increasing the inlet nozzle throat area reduces the gas inlet velocity. If the velocity referred to as "Inlet Nozzle Velocity" is the velocity at the nozzle inlet then it does not make sense that increasing the throat area will reduce the velocity at the nozzle inlet. It should be the nozzle inlet area rather than the throat area. Is that correct?
4. How is the molecular weight of the gas related to the maximum allowable flow through a nozzle (refer to page 218 in the above article). Why does that author say that higher molecular weights reduce the allowable flow through a nozzle?

On a separate note, I have seen one senior rotating equipment engineer state that increasing the number of static vanes at the first impeller inlet, reduced the "Inlet Nozzle Velocity". I have no clue how are these two aspects are related. Can someone explain this statement? I believe that the inlet guide vane is usually installed right before the impeller eye of the first impeller stage only (i.e. not ahead of subsequent impeller eye stages) and not inside the suction nozzle.

Article:

https://oaktrust.library.tamu.edu/bitstream/handle/1969.1/163407/Vol27018.pdf?sequence=1&isAllowed=y

Thank you in advance.

 

[rotw]

1 & 2 / Optimum condition of flow velocity at flange inlet ensures that the flow approaching first stage is smooth enough and within limits of inlet scroll design (including pressure drop profile vs. design and off-design flows). At design (certified) point, excessive velocities impacts predictability. Max is usually 30 ~ 35 m/s at design flow.
3/ Downstream the inlet flange, it is the "OEM business" how to optimize inlet nozzle selection and design for best performance.
So velocity limit 30 ~ 35 m/s is set at flange inlet and this is the specification for the user.
4/ Higher mol. weight could put compressor into choke even before reaching nozzle flow limit above stated.

If you plan an escape, you must succeed as if you fail, you will be punished for trying. Never say or write down your plan. Heart is the only place where secrecy is granted.

 

[Rha257]

rotw,

Thank you for being active with very helpful responses.

My understanding from your response is that the Inlet Nozzle Velocity which should be limited to 30-35 m/s is the inlet flange velocity before the gas stream goes through the nozzle.

1. This implies that the velocity of the gas stream is much higher at the nozzle throat and at the entrance of the inlet scroll. Is that understanding correct?

2. What is your opinion on the above-quoted statement regarding the number of static guide vane and their impact on nozzle velocity?

3. Also, I remember reading an amendment to API 617 specification and it was stated that the Mach Number should never exceed 0.85 at any location within the compressor unless a CFD analysis is performed to show that stalling at the impeller eye rim does not occur at higher Mach numbers. I thought this is counter-intuitive since higher Mach numbers are typically related to choking instead of stalling. What is your opinion on this?

I am sorry for bombarding the threads with a lot of question but I believe this content is very helpful for many people especially those who in the intermediate level in their careers.

 

[georgeverghese]

The reasoning for your query (4) is found in equation (4) on page 220 of the article attached. Note that the value of R, as expressed here, gets lower with increasing gas mol wt. The aim of the author here is to maintain the max gas velocity at the inlet nozzle at Mach 0.132 for all gases.

 

[rotw]

The reasoning for your query (4) is found in equation (4) on page 220 of the article attached. Note that the value of R, as expressed here, gets lower with increasing gas mol wt. The aim of the author here is to maintain the max gas velocity at the inlet nozzle at Mach 0.132 for all gases.

Absolutely spot on.

If you plan an escape, you must succeed as if you fail, you will be punished for trying. Never say or write down your plan. Heart is the only place where secrecy is granted.