22 replies to this topic

[m42364236k]

Dears

i want to know that in which cases pumps operated at end of curve and the motor of the pump shall be design based on end of curve.

as i know when we have pump  with auto start sequence,in this situation pump shall be sized based on "EOC"

what about another case?

[fallah]

as i know when we have pump  with auto start sequence,in this situation pump shall be sized based on "EOC"

what about another case?

 

Hi,

 

In all cases in which there might be a low resistance to flow such that causes the system curve will intersect the pump curve at or below "EOC" point...

[m42364236k]

thanks fallah

it meens that in all condition we should design the system with end of curve operation.

such as two pump in paralell or pump with control valve and,...

am i right ?

[S.AHMAD]

Hi

1. A good design is to have the pump operated at the BEP or as close as possible to BEP within +-10%

2. First of all you must develop the system curve then select the right pump that meet the design specification such that the system operated at near BEP.

3. However, if you have existing pump, design the system (piping etc) such that the system is operated near BEP..

4. If you design the system at the end of the pump curve, you are wasting energy

Edited by S.AHMAD, 05 January 2014 - 08:53 AM.

[fallah]

it meens that in all condition we should design the system with end of curve operation.

such as two pump in paralell or pump with control valve and,...

am i right ?

 

Hi,

 

No, you should size the pump's motor for EDC just if there would be the possibility of pump working at this point e.g. for auto starting the standby pump in parallel to the main pump...

[Jiten_process]

As Mr. Fallah said, you have to size pump for EOC operation for any possibilities that may result in lowest resistance in system. So the next task is you have to list down all such possibilites by carefully examine your system process. 

 

One of the possibility from them is, check in your system configuration, if you have discharge flow control valve in your system configuration and if that is  "FO(fail open)" type because of the process philosophy OR IF your recycle flow line has a flow control valve which is "FO" type, then your pump might experience End of curve situation and you need to size for that. 

 

Second possibility, If your system is closed loop system. It is recommended to size pump for EOC operation. 

 

Hope this helps. 

Edited by Jiten_process, 06 January 2014 - 01:06 AM.

[S.AHMAD]

Hi

1. My suggestion is you optimize the system design. Design for end of curve will increase the investment cost and higher operating cost.

2. Please give me good reasons for designing at end of curve capacity.

3 Try to understand the relationship between pump curve and system curve. Once you understand the relationship, then you will understand the reason of my suggestion.

[m42364236k]

thanks a lot  it was very useful

[Lavi]

Sharing a little bit of my knowledge.

 

Putting the pumps into EOC category is a little tricky thing and requires a sound knowledge of 'System Resistance'. I would however sum up a few general scenarios wherein rating the motors for EOC is taken as a good practice.

 

Reasons of Operating at EOC Point

 

01. Auto-start pumps (Control Logic Activated & not Stand-by Pumps)

 

Reason: These pumps normally start with discharge valves in full open condition. Hence, the flow running through the pump is high. Therefore, the operating point shifts towards our right. But there is a catch here. We need to observe our system resistance curve. If in case our system resistance curve is 'curvy' or 'half parabolic', it is very likely that the pump's motor need not be designed for EOC operation even though our flowrate is very high. Because the point of intersection of H-Q curve and System Resistance curve may not be towards the EOC. On the other hand, if our system resistance curve is flat, we need to put the pump under EOC operation category. This is well explained by Fallah Sir in post numbered 2 and 5.

 

EOC operation may be necessary for sequence/logic activated Autostart pumps only. And not Standby pumps. Please note that this is an interesting debatable topic. There are contrasting views on it. But I would however suggest that auto-starting standby pumps need not be categorised as EOC operating pump. All the pumps are equipped with a non-return valve and hence, the stand-by pump shall never reach end of curve operation. Let me explain, the stand-by pump is always flooded. On the other side of the non-return valve, there would be a back pressure. When this pump auto-starts, it shall be running with a blocked outlet condition until it builds enough pressure to open the check valve. Some people are of the opinion that the backpressure is reduced to nil as soon as the running pump trips. But it must be noted that this reduction in pressure is just momentary. And hence, it should not be much of a problem.

 

02. Pumps operating without a discharge control valve

 

Reason: Same reason again. No restriction at the discharge makes the flow higher. That is to say that back pressure is lowered and this in turn makes the operating point shift towards the right.

 

03. Pumps in parallel operation

 

Reason: Parallel pumps for EOC operation is also an interesting topic. Please refer the attachment (Source: www.chemicalprocessing.com, Article: Keep Parallel Pumps Inline, Author: Andrew Stoley, Date: Feb, 13^th 2008).

As we can see that the H-Q curve is flatter when both pumps are running simultaneously. If in case one of the pumps, is made to increase the flowrate,  the operating point would be shifted towards the right of the FLATTER curve. Maybe the combined flowrate would not be increased to such an extent that it would reach End of FLATTER Curve. But this little shift on the FLATTER curve would mean that there is a huge shift on the STEEPER curve which represents the EOC operation of the pump whose flowrate was lesser among the two parallel pumps.

 

04. Pumps with Level Control:

 

Reason: A level change in the suction tank is also hugely responsible for shifting the curve towards EOC point. A higher level would mean lower static head on the pump. A lower static head would mean shifting of operating point towards the right of the H-Q curve.    

 

Consequences of Operating at EOC Point

 

01. It would result in procurement of a motor with a high capacity. This was highlighted in post numbered 7 by Ahmad Sir.

 

02. It can also result in Cavitation if not taken care at the time of designing. This is because a higher flowrate (Operation at EOC point) is significantly going to increase our NPSHr, as these two are directly proportional. But this shouldn't be a problem until the Operation at EOC is momentary. A bit of sin is always allowed. Isn't it?

Attached Files

•  Parallel Pumps in Operation.gif   97.65KB   74 downloads

Edited by Lavi, 08 January 2014 - 03:11 PM.

[S.AHMAD]

Lavi

1. Good efforts to explain your view

2. The attached graph of yours is incomplete. You should also include pump efficiency, power, NPSr and system curves then you understand better. Furthermore, your graph does not explain the need to design EOC case. You are assuming horizontal system curve which is not a typical pumping system. To make things more clearer, you should also show several cases of system curve at several line size. Then you can see the reasons of my proposal.

3. The pump curve is for pump in series. What about pump in parallel?

4. If you plot the system curve, you should be able to determine the maximum flow when all valves are fully open. Then this maximum flow is the real maximum flow which is not necessarily at EOC case. You should choose the line size such that the operating point is at BEP that is at the maximum efficiency point.

Edited by S.AHMAD, 07 January 2014 - 03:10 AM.

[Lavi]

@ Ahmad Sir,

 

Everytime you appreciate me, I realise I have goofed up somewhere.

 

Anyway, attaching a realistic curve straight from my plant.

I guess, it is still incomplete from your point of view.

Vendor has done his job but I haven't drawn the system curve on it.

 

As per my understanding, first we generate our system resistance curve. We do this using different flowrates.

I do it for three flowrates. (Minimum, Normal & Maximum). I submit it to the vendor with an inquiry.

 

The vendor looks into my system curve. And then he proposes one of his already existing models which is a best fit for my system.   

 

Now I believe, the vendor is suggesting me one of his models based on my Normal Flowrate. I also believe this flowrate would be corresponding close to the pump's BEP. Isn't it?

 

Then I would approve his model by looking if there is enough margin considered on the right of the Operating Point (when pump is operating at normal flowrate). API-610 defines the end of the curve at 120% of BEP flow.

 

In such cases the pump has to be ready for a higher flowrate disturbance which will drag the Operating Point towards the right, or towards the EOC in my mind.

Isn't it? Please correct my understanding if wrong.

 

 

 

Attached Files

•  Doc1.doc   397.5KB   126 downloads

[S.AHMAD]

Lavi

1. Your thinking is already right but you did not show the system curves at maximum flowrate. Could you let me know how do you arrive at the  maximum system curve?

2. Maximum system curve is developed by assuming maximum opening (100%) the flow control valve. The motor size should be based on this maximum flowrate that is the meeting point of the pump curve and system curve which is not necessarily at the EOC. Of course there is possibility that the maximum flowrate is at EOC but it is dictated by the system curve. You can change the system curve by changing the line size. As a matter of fact you should draw the maximum system curve at various standards line size and hence you can select the appropriate line size for the system

3. Using the normal flowrate, you can determine the pump head and the system head. The difference between the head gives the pressure drop required by the flow control valve. Calculate the control valve C_V and select the valve that gives 70~80% opening.

4. similarly for the minimum flowrate. In fact you give this information (valve pressure drop at min, norm and max flowrate) to control valve vendor they also can recommend the right control valve for your system

5. I believe that you have the knowledge but you did not express it here

[rychurek]

Hi

Lavi

Is it right: that decreasing suction tank level cause pump flow rate to increase???

"Pumps with Level Control:

Reason: A level change in the suction tank is also hugely responsible for shifting the curve towards EOC point. A lower level would mean lower static head on the suction of pump. A lower static head would mean shifting of operating point towards the right of the H-Q curve."

[fallah]

Is it right: that decreasing suction tank level cause pump flow rate to increase???

"Pumps with Level Control:

Reason: A level change in the suction tank is also hugely responsible for shifting the curve towards EOC point. A lower level would mean lower static head on the suction of pump. A lower static head would mean shifting of operating point towards the right of the H-Q curve."

 

Hi,

 

Keeping the other parameters constant, lower total differential static head (discharge static head minus suction static head) will result in shifting the system curve toward right side of the pump curve.

[Lavi]

@ Ahmad Sir:

 

I am understanding what you are trying to explain. And I agree with your points. You want to say that I will have to reconsider my line sizes once I have fixed my pump model. Depending on the pump model, I would change the slope of my system curve by changing the line size and hence the system curve would never operate close to EOC region (in most of the cases). I would preferably keep the line sizes lower as this would make my system curve move away from EOC region. Higher line sizes would flatten my system curve.

 

And I thank you for correcting me, I was under the assumption that motor is sized on Normal flow rate and not Maximum flowrate. This was a learning today.

 

@ Rychurek:

 

There has been a typo. It has already been corrected by Fallah Sir. Please read the edited version again. And also note that the explanation is in reference to a situation wherein level is controlled with a valve on the discharge side of the pump.

 

Coming to your question, if decreasing the level in the tank would increase the flowrate.

 

No, it shall not increase the flowrate. In fact, for a fixed speed centrifugal pump the flowrate and corresponding head is always going to remain constant until we don't exploit its performance using additional features. It is unusual for a system to require operation at a single fixed flow rate. And it is unusual for a pump to provide the system with varying flowrates.

 

Consider this example for clarity:

 

I have to deliver a volume of liquid, say about, 50 m³/hr to one of my destinations. Unfortunately, I do not get the perfect pumping model and the vendor suggests me a model which can deliver about 60 m³/hr of liquid constantly. So, in this case, I shall be using a control valve at the discharge piping of the pump and reduce the flowrate by 10 m³/hr. In turn, I would be also be killing some pressure and increasing the head. (Refer Bernoulli, Pressure Energy getting converted to Potential Energy and Velocity Energy). I guess you can now co-relate this explanation with the H-Q curve.

 

Also note that, closing the valve increases control losses and causes the system head curve to slope up more steeply to intersect the H-Q curve at the desired capacity. Opening the valve decreases the control losses and causes the system head curve to slope downward and intersect the H-Q curve at a higher capacity.

[S.AHMAD]

Dear Lavi,

1. It is ok for you to size the motor based on normal flowrate if you start the pump with discharge valve close. However we normally add 10% safety factor

2. The size to maximum flowrate is for starting the pump with discharge valve open.

[rychurek]

Lavi

Sorry it was my misreading. That's why I wrote: decreasing tank suction level affect flow to increase. In your post nr 9 it was higher liquid level not lower liquid level.

Best regards Richard

[roshimessi]

dears as 

i am graduated chemical engineering student ,

i have some question

i will be appreciated if you help me

as i understand 

1- for installing the pump in service in first step of project

as i understand in this topic, first we should calculate the pressure drop of the system that we wanted to put the pump

for example if pressure drop of the system from vessel to another vessel was  2bar we should put the pump to overcome this pressure drop is not it?

2-in the system for example from the vessel to other vessel we have just normal  flow rate what is the other flow rate (minimum and maximum)because in the first of the project how we can find min and max\flow rate?(you mean min and max shall be calculated based on the level of the tank?

3- h-q curve of the pump is the” differential pressure vs flowrate “or “ discharge pressure vs flow rate“and as i know for define the suction pressure of the pump we consider the  suction drum in low low liquid level,if the vessel put in high high liquid level  it means that the differential pressure of pump decrease because the discharge pressure is constant and suction pressure is increased and it is\the cause to have more flow rate due to this differential pressure is not it?

[roshimessi]

Dar lavi as jiten saied

When we have control valve in discharge of the pump in "FO" ,due to process fault,operating curve  movie to right ,what about "02. Pumps operating without a discharge control valve"

without control valve why the pump should be sized based of EOC,as mr ahmad said for starting the pump we start the pump by closed discharge

Please correct my understanding if wrong.

[roshimessi]

Dar lavi as jiten saied

 

When we have control valve in discharge of the pump in "FO" ,due to process fault,operating curve  movie to right ,what about "02. Pumps operating without a discharge control valve"

 

without control valve why the pump should be sized based of EOC,as mr ahmad said for starting the pump we start the pump by closed discharge

 

Please correct my understanding if wrong.

[Lavi]

@ Ahmad Sir,

 

Agreed to both your points and updated myself. Thank you once again.

 

@ Messi,

 

Welcome to the community.

 

For your general questions, find the answers below:

 

1. Yes, you have got it correctly.

 

2. Minimum and Maximum flow rates are defined in the project design basis/philosophy as required by client. Generally,
          Minimum Flowrate = (0.25 or 0.30) * Normal Flowrate
          Maximum Flowrate = (1.05 to 1.50) * Normal Flowrate
It entirely depends on client's requirement.

3. H-Q curve is Head v/s Capacity curve. By head we understand it as Total Dynamic Head (TDH) of the centrifugal machine (not just a pump). TDH is calculated using,
          TDH = (Elevation Difference) + (Frictional Losses) + (Pressure/rho*g) + (velocity²/2g)

 

For conditions with zero flow,

          TDH = (Elevation Difference) + (Pressure/rho*g)

 

 

For question directly asked to me, read the explanation below

 

As mentioned by you in point 3, for conservative results while designing the "pump we consider the suction drum in low low liquid level" In other words, you consider a high Elevation Difference (Differential Head) between suction and discharge while designing. But imagine a situation wherein there is no control valve and the level in the suction drum goes high during a process upset, making the Differential Head smaller. In these cases, the pump will suddenly move towards EOC region. (And not necessarily at the EOC point).

 

As you have noticed, Ahmad Sir has correctly pointed out that such type of scenarios can be avoided by manipulating the line size.
 

Please don't hesitate to ask more questions. There are many people out here who can help you to better your understanding.



 

[roshimessi]

dear lavi 

 

thank you for your good help

 

as i understand ,without control valve pump suddenly move toward EOC,

 

what about the  pump with control valve with FO position in this situation if the valve suddenly go through full open in this situation we should considered for EOC

[S.AHMAD]

 

Messi,

 

I will try to answer your questions as per your numbering:

 

1- for installing the pump in service in first step of project

As i understand in this topic, first we should calculate the pressure drop of the system where we want to put the pump.

For example, if the pressure drop of the system from vessel to another vessel was  2bar we should put the pump to overcome this pressure drop is this not it?

 

For a new project we normally do not have a pump as yet, but we need to provide information to the pump vendor so that they can make the right recommendation of pump that can deliver the volume of flow (we called pump capacity) that we require. One most important information is the one that we call the "System Head" at the normal capacity/flowrate. The pump vendor normally recommends a pump such that the pump is operating at near best efficiency point (BEP). The system head is calculated by taking the energy difference between the destination point (let say point 2) and the source point (let say point 1):

 

System Head = (P₂ - P₁)/rho. g  + (Z₂ - Z₁) + 4f(L/D)V²/2g

Where

P = pressure, in Pa

Z = elevation (m)

V = velocity (m/s)

f = fanning friction factor

L= equivalent length (m)

D = diameter (m) and

g = 9.81 m/s².

 

2-in the system for example from the vessel to other vessel we have just normal  flow rate what is the other flow rate (minimum and maximum)because in the first of the project how we can find min and max\flow rate? (you mean min and max shall be calculated based on the level of the tank?)

 

Minimum and maximum flow could be the process  requirement (e.g plant operate at 50% capacity). Under this scenario, normally flow control valve will be provided to control the flowrate. However, the pump vendor may also dictate the minimum flow requirement to prevent vibration that can lead to pump damage. This minimum flow is determined by real/actual test in their workshop.

 

3- h-q curve of the pump is the” differential pressure vs flowrate “or “ discharge pressure vs flow rate“and as I know for define the suction pressure of the pump we consider the  suction drum in low low liquid level,if the vessel put in high high liquid level  it means that the differential pressure of pump decrease because the discharge pressure is constant and suction pressure is increased and it is\the cause to have more flow rate due to this differential pressure is not it?

 

H-Q curve - there are two type H-Q curves. Pump curve that is provided by the vendor and system curve that we need to develope as per my answer no. 1 above. The unit for H is in m (or ft) and flowrate in kl/h (or gpm). Pump curve s not affected by liquid levels. However system curve as shown above (answer 1) is affected by pressure, liquid levels (that is the elevation) and diameter of the piping. The meeting point between these two curves give the operating point of the system. If the system curve for 100% opening of valve, the operating point corresponds to maximum flowrate. For good design, the normal flowrate capacity should be as near as possible to BEP at 70~80% control valve opening