Surge in Centrifugal Fans/Blowers (API 673/ API 560) 1

[CuriousMecEng]

Would like to hear your opinions on the below points regarding the surge in centrifugal fans/Blowers. (API 673/ API 560)

1. Is surge a major issue in low-power(<50 kW) centrifugal fans?
2. Is it bad or not preferred to operate on the left side of the surge line for such a blower?
3. What are the effects if operated on the left side of the surge line?
4. Considering a turndown requirement of less than 5% of rated flow, is this acceptable?


Thanks in advance.
Stay Curious!!

 

[georgeverghese]

You may ruin your fan/blower if you operate within the surge zone.
For an operating point of 5% of rated flow, if the blower/ fan is still within the surge zone, install a min flow recycle line. If this is in air service, the min flow could direct excess air to open air. You can reduce power demand at min flow by installing VFD on an electric motor drive.

 

[CuriousMecEng]

@georgeverghese: Thank you for the reply.

1. Practically, How severe is surge and stall for a 25-30 kW (BHP) centrifugal fan/blower (backward vane, aerofoil)?
2. Over a long-term usage, what are the implications of running the blower on the left side of the curve?

Thanks in advance.

 

[LittleInch]

Curious...

No one will know. It could be fine or the machine even at 25 to 30kW could vibrate violently. 25kW is still a lot of power and when it goes wrong can destroy this very quickly.

Long term use at the LHS is OK so long as you are not beyond the surge line. It's just normally a bit inefficient.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[CuriousMecEng]

@LittleInch: Thank you for your reply.

Please do correct me if my understanding is wrong.
1. Please provide clarity on the statement "Long term use at the LHS is OK so long as you are not beyond the surge line". Do you mean not moving the operating point towards the right of the surge line?
2. If my control is through a damper downstream, and my operating cases require the blower to operate both at the right and left of the surge line, what are the impacts on the blower?

Note that VFD cannot be used, as my pressure requirements are the same for both cases. (Operating and turndown)

Thanks in advance.

 

[LittleInch]

Sorry, I thought left hand side meant low flow but more that the surge line.

Operating to the left of the surge line is not recommended and could very easily destroy your machine. But it might not either. Depends on the individual machine.

The impacts on the blower could be catastrophic. You really should have an anti surge system set up to maintain a minimum flow via a recycle or dump system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Even on a low 50kW machine, these vibrations can do damage to the shaft bearings and especially shaft labyrinth type seals - I have not heard of any one operating a centrifugal machine inside the surge zone. If you still have doubts, the blower manufacturer can also advise. Running the blower to the far right doesnt do any mechanical damage, but machine operation is inefficient.
If you have a damper for flow control to the downstream side, install a blow off valve (for air service) operating on flow control upstream of the damper. The FT for the FIC should be upstream of the piping tee leading the FCV. The fail open FCV should preferably be quick open trim. FIC setting should be at least 10% higher than surge flow at the highest expected operating pressure. Tuning of this min flow FIC can be critical if you expect fast operation of this control damper downstream. Aim to keep control damper operation gradual if possible.

 

[CuriousMecEng]

@georgeverghese and @LittleInch: Thank you for all your inputs. I really appreciate taking the time in responding to the query.

1. If the blower OEM is accepting these operating points (Left and right of surge line) and assures satisfactory working of the blower at these points (operating and turndown), How does an end-user validate this?

Does the end-user have to rely on OEM experience?

Thanks in advance.

 

[georgeverghese]

Can we see the Q-h curve for this blower? Is the the delivery pressure less than 1.5psig ?
The definition of surge itself implies high frequency flow reversal leading to high vibration. If there is a surge zone for this machine, it is strange the manufacturer has no issues with long term operation inside this zone. In any case, there is no harm in installing a min flow control loop.

 

[CuriousMecEng]

@georgeverghese :
Sorry, I cannot share the curve. However, the delivery pressure is 1.55 psig.
The application is for burner combustion air. What are the chances of flow reversal for this application?

 

[georgeverghese]

If the manufacturer Q-h curve indicates a surge point for a given speed, then high frequency flow reversal cycles are implied at flows within the surge zone.

 

[georgeverghese]

From memory, I cannot recall seeing a min flow blowoff line operating on FIC on FD combustion air fans either ( on about 3-4 units of fired heaters with FD combustion air centrifugal fans). Didnt occur to me that these blowers could be operating at flows within the surge zone at that time. Years after these were put into operation, havent heard of any problems with these. So your manufacturer may be right.

 

[CuriousMecEng]

@georgeverghese: Thank you for all your input.

 

[SB1976]

Hi,

[Q-1] Is surge a major issue in low-power(<50 kW) centrifugal fans?
Ans. You need to understand the difference between a fan / blower and a compressor. In case of a "fan" or a "blower", pressure rise is relatively small (1.05~1.15) and hence flow across the fan can be considered incompressible (just like a pump). A compressor handling compressible fluid is associated with very high pressure rise (>1.2) which raises the temperature of the pumped fluid. Thus, we have "surge" phenomenon in a compressor, marked by a high discharge temperature. As such, for low power centrifugal fans, there is hardly any temperature rise and hence surge is not a very critical issue.

[Q-2] Is it bad or not preferred to operate on the left side of the surge line for such a blower?
Ans. Operation on the left side of the surge line is detrimental to the life of the rotating machine concerned.

[Q-3] What are the effects if operated on the left side of the surge line?
Ans. Reduced bearing life, high vibration.

[Q-4] Considering a turndown requirement of less than 5% of rated flow, is this acceptable?
Ans. No. The means of inherent turndown methods (such as a scoop tube, VFD, fluid coupling etc.) do not support such high turndown for a CF fan. This is because of elevated losses in the system due to low speed operation.

Regards.

 

[compguy22]

I agree with LittleInch.

It is not advisable to operate at flow less than surge flow plus a certain margin. Anti-surge protection should be implemented.
The magnitude of damages or alteration and negative impact on lifetime may vary, irrespective of the size.
Certainly, surge in bigger machines with higher compression ratio and power density may exhibit more "violent" behavior with possible but rare cases of loss of containment; all in all if a smaller machine ends up non operational, consequences are of same nature: cost of loss of production, relatively costly repairs and replacement of main equipment and possibly elements of the system.

 

[CuriousMecEng]

@SB1976 : Thank you for your response.
a) In point 1, you have mentioned " As such, for low power centrifugal fans, there is hardly any temperature rise and hence surge is not a very critical issue.": Can it be inferred that low-power fans can be operated at the left of the surge point without reliability issues?
b) Regarding point 4: What are the options for such a requirement of up to 5% turndown in fans?
b) Can you support in understanding the technical reason behind the comment by "georgeverghese (Chemical)5 Jun 22 08:05
"From memory, I cannot recall seeing a min flow blowoff line operating on FIC on FD combustion air fans either ( on about 3-4 units of fired heaters with FD combustion air centrifugal fans). Didn't occur to me that these blowers could be operating at flows within the surge zone at that time. Years after these were put into operation, haven't heard of any problems with these. So your manufacturer may be right."

 

[CuriousMecEng]

@compguy22 : Thank you for your response.
Based on your experience, how detrimental can the reliability of low-power fans (<50 kW) running at the left of the surge point be? Specifically for combustion air fan applications.

 

[LittleInch]

Curios,

I'm curious as to why it has taken 4 months to come back to us and still asking the same questions?

Anyway, I can't see how we can answer your questions, which arise as far as I can see from discrepancies and conflicts in the information you're getting from the vendor.

Re reading this it seems that the vendor has supplied a curve with a surge line. This strongly implies that you should not operate to the left of this line.

Ob the other hand you appear to have some correspondence / information from another part of the vendor organisation saying it is OK to operate to the left of the surge line without surge relief / recycle line.

Operating at 5% flow is very low. What sort of percent of the time does it operate like this? If it's more than say 5% of the time, then you really should install a lower flow fan / blower and then swap over when you need more flow.

So the only way you're going to get an answer is to get this IN WRITING from the vendor and then if / when your fan bearings fail you have some sort of recourse.

Or just accept the risk. Do you have a spare? What happens if the fan fails / starts making nasty noises?

Only you can make these judgements as each type of fan and usage will be different and a different risk "appetite".

IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Pls note that in the subsection for blowers and fans with Pd<1.5psig in Perry Chem Engg Handbook, there is no talk of surge or stonewall operation and neither is a surge line shown in the generic Q-h curves for these machines. In the subsequent section for centrifugal compressors, it says that the operating range in flow is reduced when (a) pressure developed per stage is high or / and when (b) no of stages is high. In these blowers and fans, there is only one stage and pressure developed is low.

 

[georgeverghese]

Also note that the air-fuel ratio controls configuration also affects the operating range of the FD fired heater fan flow. If the burners are ALL operating in parallel, then typical fuel flow turndown is 5:1 and so will the air flow which tracks fuel flow. So operating fan flow will not drop below say 15-20% of design flow, including during startup.

If the burners are staged with a high capacity and a low capacity burner, then turndown will be approx 25:1 on FD fan flow also if air-fuel ratio control is used. In this case, it may be preferable to use a 2 speed fan (or motor operating on VFD) rather than to implement min flow to blowoff to save on power, (and also avoid running this fan almost at dead head) if this savings is of value in your plant. In this case, the fan Q-h curves at various speeds must be suitable for speed variation as a means of deriving variable flow.