33 replies to this topic

[Julien123456789]

Dear all,

 

I am a freshman in refining processing.

I would like to know what is the discharge pressure of a positive displacement pump when there is no pipe at outlet ?

 

Your explanation will be strongly appriciated.

 

Thank you in advance.

[fallah]

 

I would like to know what is the discharge pressure of a positive displacement pump when there is no pipe at outlet ?

 

 

Julien,

 

Please avoid several posting the same thread...

 

When there is no pipe at PD pump, discharge pressure would be atmospheric pressure...

 

Actually, to avoid liquid free-flowing through a PD pump, there should be a greater pressure at the discharge than the pump suction...

[Julien123456789]

Thank you so much, Fallah.

I tried to delete my 2nd post but i dont know how to do it.

 

Have a nice day,

[fallah]

Thank you so much, Fallah.

I tried to delete my 2nd post but i dont know how to do it.

 

Have a nice day,

 

Thanks Julien,

 

It could be deleted by a hidden point at the left of the "MultiQuote" botton...
 

Edited by fallah, 04 April 2016 - 06:52 AM.

[Julien123456789]

Thank you Fallah,

Perfectly, i did it !

[breizh]

Hi Julien ,

 

this document may help you .

http://www.warrenpum...ces/pdpumps.pdf.

 

Good luck

Breizh

[Julien123456789]

Thank you so much, Breizh,

I am trying to read this document.

 

Have a nice day !

[fallah]

Julien,

 

One more point: In the cases in which the dischrge pressure might be fallen below the minimum required pressure, in order to keep the pressure at discharge a back pressure valve should be applied...

[Julien123456789]

Julien,

 

One more point: In the cases in which the dischrge pressure might be fallen below the minimum required pressure, in order to keep the pressure at discharge a back pressure valve should be applied...

 

Thank you Fallah for this remark.

This method are both  applicable for centrifual pump and positive displacement pump ?

Am I right ?

[fallah]

 

This method are both  applicable for centrifual pump and positive displacement pump ?

Am I right ?

 

 

Julien,

 

Using back pressure valve is mostly applicable to PD pumps...

[Julien123456789]

Thank you for your clear explannation !

[Julien123456789]

 

 

This method are both  applicable for centrifual pump and positive displacement pump ?

Am I right ?

 

 

Julien,

 

Using back pressure valve is mostly applicable to PD pumps...

 

 

Dear Fallah,

In the following sketch, which is the inlet pressure of the vacuum reactor ?

I am trying to understand an example about your remark.

I got confused because i saw the set point of the back pressure valve =  35 psia. On your previous post, you told me that the outlet pressure of pump is always equal to its of destination.

 

 

The 2nd thing, in this sketch, we can control flow and pressure that enter to the vacuum reactor at the same time ?

Is it complicated for sizing the bypass control valve ?

 

Thank you Fallah ! 

Attached Files

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Edited by Julien123456789, 04 April 2016 - 10:45 AM.

[fallah]

 

In the following sketch, which is the inlet pressure of vacuum reactor ?

I am trying to understand an example about your remark.

 

 

Julien,

 

The inlet pressure of the reactor is 1 psi, while the upstream of the back pressure valve will be kept at 20 psig...

[Julien123456789]

Thank you Fallah,

However, the back pressure valve is not used for assuring a constant inlet pressure for the vacuum reactor or at least a constant pressure at the back pressure downstream ?

I think it is a good purpose when using this type of valve.

May I get confused between a back pressure valve and back pressure regulation ?

 

Please let me know if i am wrong.

Edited by Julien123456789, 04 April 2016 - 10:59 AM.

[fallah]

 

However, the back pressure valve is not used for assuring a constant inlet pressure for the vacuum reactor or at least a constant pressure at the back pressure downstream ?

 

May I get confused between a back pressure valve and back pressure regulation ?

 

Please let me know if i am wrong.

 

Julien,

 

The back pressure valve is mainly used to keep the pump's discharge pressure, in your case, not being lower than 20 psig; even the operating pressure of downstream equipment i.e. the reactor to be at a very lower pressure (1 psia)...

[Julien123456789]

Thank you for your explannation.

I see what you mean and i try to understand it.

 

Thank you one more time, Fallah,

 

Julien.

[fallah]

Julien,

 

A back pressure valve is a normally open valve which is going to be closed once upstream pressure tends to be lower than a prespecified value. Then it will be partially or even completely closed to keep the upstream pressure higher than that value. Eventhough the pressure of the liquid will be reduced through passing a back pressure valve it's not named as pressure reducing valve at all. If it's not being installed at PD pumps with potential of lower discharge pressure than suction pressure; the fluid will flow freely to the destination without any control and limitation which would be in contradiction with the concept of a metering pump...

[Julien123456789]

This is a very clear explantation.

I understand what you mean. I got confused between back pressure valve and pressure reducing valve.

Thank you so much Fallah,

 

Have a nice day,

 

Julien

[shantanuk100]

Dear Julien,

 

Just a little additional information to add to the detailed explanation Mr. Fallah has given.

 

1. There are basically two types of ways you can regulate the pressure in these sort of cases : one is by using a pressure reducing valve/regulator and the other is by using a backpressure valve/regulator, which is your case.

 

2. In a pressure reducing valve, the pressure that is being controlled is the pressure which is downstream (After) the valve. Once this pressure downstream is increasing close to or beyond a set-point, then a spring/feedback mechanism is triggered for the valve to open. Once the pressure downstream (after) the valve falls below the set-point, the valve/regulator recloses and normal operation continues where the fluid passes through the valve as usual.

 

3. There might be certain pressure reduction of the fluid upon passing through the valve/regulator, but this is not the purpose of the regulator. The main purpose of the valve/regulator in these cases is to maintain a steady flow, without disturbance, based on whichever pressure (upstream or downstream) is being monitored.

 

4. In the other type of valve which is a Back pressure regulator/valve, you are monitoring the pressure which is upstream of the valve. Once the pressure upstream (before) the valve falls below a certain set-point, the backpressure regulator is triggered to close and when there is a pressure increase upstream, beyond the setpoint, it starts to go into the open position.

 

5. If you visualize the purpose of each valve, you'll understand. In regular pressure reducers, the valve has to relieve some pressure in the process fluid, so the only option is for it to open when the process fluid pressure exceeds the spring pressure. In a backpressure regulator, the purpose is to regulate the backpressure on the valve.

 

6. Also for further detailed explanation if required, I would suggest using the following resource.

http://www.processin...astOMaticWP.pdf

 

 

Cheers,

Shantanu Kallakuri

Edited by shantanuk100, 05 April 2016 - 10:56 PM.

[fallah]

 

4. In the other type of valve which is a Back pressure regulator/valve, you are monitoring the pressure which is upstream of the valve. Once the pressure upstream (before) the valve falls below a certain set-point, the backpressure regulator is triggered open and when there is a pressure increase upstream, beyond the setpoint, it falls back to close position.

 

 

Shantanu,

 

Appears the valve action to be vice versa; i.e. once the pressure upstream falls the valve is triggered closed and when the pressure increases it tends to be opened... right?
 

[Julien123456789]

Shantanuk,

Thank you so much for your clear explanation.

In a metering pump application, I have impression that they want to maintain the upstream pressure of the back pressure valve. 

Some literature explain  the metering pump operating point does not change a lot in that way. Therefore, it can improuve the accuracy of metering.

The 2nd thing, Mr. Fallah already explained on the post # 17 that a back pressure valve may be used to avoid the siphoning effect when a suction pressure is higher than a discharge pressure.

 

I am so happy for having finding this forum. You are all experienced person and mostly you are kind !!

 

Have a nice day,

 

 

Julien

[shantanuk100]

@Fallah,

 

Thanks Mr. Fallah for pointing that out.

I hadn't noticed the typo with so many open and close terms.

I've just corrected it in the original post too.

 

@Julien,

 

1. Yes, as Mr. Fallah had pointed out, what happens in siphon effect is that there are certain cases where the suction pressure becomes too high. So when that happens, there is a creation of vacuum temporarily, and a lot of fluid from downstream has a possibility of being pulled back in order to fill up that vacuum.

 

2. In order to avoid this, you need to regulate the pressure upstream of the valve and ensure a steady change.

That is why we need to use a BPR over there.

 

3. No worries. I'm sure that you would always find help over here, with the vast number of experienced professionals, like Mr. Fallah and Mr. Art, who have a lot of knowledge and experience to share.

 

Cheers,

Shantanu

Edited by shantanuk100, 06 April 2016 - 04:01 AM.

[Julien123456789]

 

 

1. Yes, as Mr. Fallah had pointed out, there are certain cases where the suction pressure becomes too high. So when that happens, there is a creation of vacuum temporarily, and a lot of fluid from downstream has a possibility of being pulled back in order to fill up that vacuum.

 

2. In order to avoid this, you need to regulate the pressure upstream of the valve and ensure a steady change.

That is why we need to use a BPR over there.

 

 

 

Dear Shantanuk,

Thank you for your contribution.

I think it is vice versa in the point 1 and 2 on your last post. It means that there are certain cases where the discharge pressure becomes too LOW. So when that happens, there is a creation of vacuum temporarily, and a lot of fluid from UPSTREAM has a possibility of being pushed out violently in order to fill up that vacuum.

 

Please let me know if i don't understand the suject.

 

Julien.

 

Edited by Julien123456789, 06 April 2016 - 01:52 AM.

[fallah]

 

I think it is vice versa in the point 1 and 2 on your last post. It means that there are certain cases where the discharge pressure becomes too LOW. So when that happens, there is a creation of vacuum temporarily, and a lot of fluid from UPSTREAM has a possibility of being pushed out violently in order to fill up that vacuum.

 

Please let me know if i don't understand the suject.

 

 

Julien,

 

"Discharge pressure becomes too LOW" is on the same track of "Suction pressure becomes too HIGH" as mentioned by Shantanu; and there is no contradiction between these two terms...

[Julien123456789]

Dear Fallah,

I am so stupid. Haha.

You and Shantanuk are right.

 

Julien.