16 replies to this topic

[gita]

We are developing a project where a couple of API 610 centrifugal pump are used to pump crude oil from a tank depot to a ship loading terminal through a pipeline (9600 m3/h @ 60 barg).

A recirculation line has been foreseen in the pumping station to pump back to the tank the crude oil if the terminal is not ready to accept the oil.

Recirculation line has been sized according to minimum pump allowable flow (at the minimum allowable pump rotating speed) and a restriction orifice has been selected in order to dissipate the fluid energy and have a covenient pressure drop.

Restriction orifice have been physically foreseen to be installed closed to the pump in order to have the advantage to use a lowe rating pipe and save some costs (as matter of facts pumps are located seveal hundred meters from the recirculation tank.

We have received a comment by our client asking us to move the orifices close to tank tie in in order to "avoid pressure build up in the recirculation line".

Our thinking is that the pressure in the recirculation line is given by the positive head in storage tank with the addition of pressure due by frictional losses. No other terms should be considered.
Or there is something that we are missing ?

Furthermore, there are some normative restrictions that binding to install the restriction ordifices in  specific point ?

Appreciating your opinion on matter.

 

[ankur2061]

Gita,

 

Refer "Scheme-7" sketch in the link below:

 

http://www.cheresour...trifugal-pumps/

 

Individual RO's in each pump discharge line should be provided if there are multiple pumps in paralel and an auto-start facility is provided.

 

If this is not the case (auto-start of multiple pumps in parallel) then a RO in the common discharge line can be provided which can be close to the tank where the liquid is being recycled. Providing the RO on the commond discharge line close to the pumps has certain disadvantages:

 

1. RO will need to be sized for a higher pressure drop if it is close to the pumps. Higher pressure drops can cause cavitation and noise.

 

2. A lower pipe rating downstream of the RO cannot be justified from a safeguarding point of view since RO's are subject to wear and tear. The normal practice is to rate the discharge piping including the forward flow piping and the recycle flow piping for the shut-off head of the pump.

 

3. If it is insisted that the pressure rating of the recycle line after the RO be reduced, then you may have to provide an additional safeguarding for the lower pressure rated piping downstream of the RO by providing a shut-down valve to close downstream of the RO in case the recycle line operating pressure exceeds the pressure rating of the piping. Additional expense of a shut-down valve and a pressure transmitter (diaphragm type) would be required. Alternatively a relief valve may be required in the recycle line to protect the piping. If a relief valve is installed it is also incurs extra cost and additionally the discharge of the reilef valve needs to be routed to a closed drain system  since we are talking about crude oil.

 

In fact I would recommend that you opt for "Scheme-6" provided in the link above if your minimum recycle flow is less than 30% of the pump design flow.

 

Hope this helps.

 

Regards,

Ankur..   

[fallah]

gita,

 

The pumps are located several hundred meters away from the tank, then buil up back pressure downstream of the orifice might be considerable if the orifice to be located near the pump. It can hinder the minimum flow passing through the orifice due to decreasing the pressure drop across it...

[gita]

gita,

 

The pumps are located several hundred meters away from the tank, then buil up back pressure downstream of the orifice might be considerable if the orifice to be located near the pump. It can hinder the minimum flow passing through the orifice due to decreasing the pressure drop across it...

 

Is Pressure build up related only to frictional losses or there is other terms that we need to taken into account ?

[fallah]

Is Pressure build up related only to frictional losses or there is other terms that we need to taken into account ?

 

 

gita,

 

It relates to frictional losses and probable liquid static head (lack of the system sketch and geometry)...

[curious_cat]

In spite of the discussion I'm still confused by the following statement:

 

 move the orifices close to tank tie in in order to "avoid pressure build up in the recirculation line"

 

What exactly does it mean?

 

Also @fallah can you elaborate on your statement:

 

 buil up back pressure downstream of the orifice might be considerable if the orifice to be located near the pump. It can hinder the minimum flow passing through the orifice due to decreasing the pressure drop across it...

 

I am confused about what you mean by "build up back pressure". 

[curious_cat]

3. If it is insisted that the pressure rating of the recycle line after the RO be reduced, then you may have to provide an additional safeguarding for the lower pressure rated piping downstream of the RO by providing a shut-down valve to close downstream of the RO in case the recycle line operating pressure exceeds the pressure rating of the piping. 

 

Which might be an interesting situation because: The RO was inserted to protect the pump, I assume? In a scenario where  such a shut down valve closes to protect downstream-of-RO piping from high pressure it will have the inadvertent cascading effect of not exposing the pump to a no flow condition. 

 

Probably a bigger disaster. 

Edited by curious_cat, 25 September 2013 - 02:06 AM.

[ankur2061]

curious cat,

 

Your half-statements on my responses do not help but just create more confusion.

 

I have categorically mentioned that the best solution is not to derate the minimum recycle flow piping and maintain the same rating for the forward flow and recycle flow piping. I have also mentioned that a relief valve can be provided if need be on the recycle line instead of a shutdown valve if the recycle flow piping is derated.

 

Also note that centrifugal pumps do not explode if there is no flow (dead-headed) for a short period of time. There is enough time to manually take action of stopping the pump if such a situation arises considering that the amount of automation available today allows complete monitoring of the pump. For your information, if a centrifugal pump runs dead-headed for a long time the bearing temperatures tend to rise and interlocks are provided to trip the pump on high bearing temperatures.

 

Regards,

Ankur

 

P.S.: BTW, are you a chemical engineer. I am asking this because "built-up backpressure" is a very common terminology in process engineering and even the relatively less experienced engineers know the definition of this term.

Edited by ankur2061, 25 September 2013 - 12:37 AM.

[fallah]

 

Also @fallah can you elaborate on your statement:

 

 buil up back pressure downstream of the orifice might be considerable if the orifice to be located near the pump. It can hinder the minimum flow passing through the orifice due to decreasing the pressure drop across it...

 

 

Hi,

 

It is very clear as Ankur mentioned. Just compare two situations: 1)orifice installed near the tank, 2)orifice installed several hundreds meters away from the tank. In the first situation the pressure downstream of the orifice will adjust itself against the pressure almost corresponding to the liquid static head in the tank. In the second one, the pressure downstream of the orifice will adjust itself against mentioned pressure plus considerable friction losses due to fluid motion inside the pipe...Then the pressure would build up to compensate...

[curious_cat]

@Ankur,

 

(1) I like & wholeheartedly agree with your "maintain the same rating for the forward flow and recycle flow piping." solution.

 

(2) I also agree with the relief valve solution as an alternative

 

(3) I do not like (but maybe I am wrong) the shutdown valve solution. I think this is worse than the original problem. (I'd love to hear if anyone has put an automatic overpressure shutdown valve on a cfg pump minimum flow recycle line. Is this common practice? )

 

As an aside, it seems to me that the likelihood of facing an over-pressure on the recycle line piping might be especially likely to correlate with occasions where the main line is indeed inadvertently blocked thereby pushing all flow to the RO / recycle? Risks  which might normally be thought to be independent are, in fact, correlated.

 

(4)  I will exclude the "manually take action of stopping the pump" scenario because if that was an acceptable route of action (which it very well might be in certain situations, no doubt) why have the RO + recycle line in the first place? All you'd need is manual intervention / automation and pump shutdown. IMHO there's still a place for passive safety measures just exactly because active measures can, and do, fail from time to time. 

 

Maybe I am wrong. I defer to your greater experience in these matters. 

 

I agree (1) being better than (2) but again (respectfully) I do not think (3) is an acceptable / feasible strategy at all.  Would love to hear other opinions, of course. 

Edited by curious_cat, 25 September 2013 - 02:15 AM.

[curious_cat]

@fallah:

 

Thank you. I think I understand what you mean. 

 

Thanks for humoring my doubt. 

[ankur2061]

3) I do not like (but maybe I am wrong) the shutdown valve solution. I think this is worse than the original problem. (I'd love to hear if anyone has put an automatic overpressure shutdown valve on a cfg pump minimum flow recycle line. Is this common practice? ) As an aside, it seems to me that the likelihood of facing an over-pressure on the recycle line piping might be especially likely to correlate with occasions where the main line is indeed inadvertently blocked thereby pushing all flow to the RO / recycle?

 

1. Minimum flow recycle lines are not sized for pump design flow, they are to be sized only for the minimum flow as certified by the pump vendor.

 

2. Having automatic shut-down valves also generally referred to as either "Process Shutdown" valves (PSD valves) or "Emergency Shutdown" valves  (ESD valves) on pumped lines is not something new. It has been practiced in the chemical process industry since long. A process upset or an emergency might require a section or complete unit to be isolated. In such case there is a trade-off in trying to protect the section / unit against a single piece of equipment. A PSD or ESD system is normally designed to ensure that the rotating equipment of the section / plant shuts down either sequentially or instantaneously depending on the plant section / unit operating philosophy and / or the type of rotating equipment.

 

Personal likes and dislikes on how things should be and shouldn't be, is obviously everybody's fundamental right to express himself or herself. I too wish a lot of process design or operations was done the way I liked it, but the fact of the matter is that certain time-tested practices have been adopted in the chemical process industry and a majority in the industry find it workable and satisfactory and are not inclined to change it.

 

I think you should be talking to some of your experienced supervisors in such matters. It seems that you have much to learn on either process design or process operations. Consulting your direct supervisors where you work in my opinion should be your starting point.

 

Regards,

Ankur

Edited by ankur2061, 25 September 2013 - 03:21 AM.

[Sengshel]

Hi,

 

Thanks for interesting discussion, however, I still do not get the difference that the position of the restriction orifice makes.

fallah gives a good example (see quote) but it seems not complete.

 

Hi,

 

It is very clear as Ankur mentioned. Just compare two situations: 1)orifice installed near the tank, 2)orifice installed several hundreds meters away from the tank. In the first situation the pressure downstream of the orifice will adjust itself against the pressure almost corresponding to the liquid static head in the tank. In the second one, the pressure downstream of the orifice will adjust itself against mentioned pressure plus considerable friction losses due to fluid motion inside the pipe...Then the pressure would build up to compensate...

 

 

option 1: RO installed near the tank:
The liquid has a high pressure at the pump, then loses some pressure due to the friction losses in the pipe until it reaches the RO, then comes the pressure drop across the RO. Subtracting any static liquid head should yield the pressure in the tank.comes the pressure drop across the RO. Subtracting any static liquid head should yield the pressure in the tank.

option 2: RO installed near the pump:
The liquid has a high pressure at the pump, then comes the pressure drop across the RO and then the pressure drop due to the friction losses in the pipe until it reaches the tank.

 

In both cases the pressure drop across the minimum flow line is determined by the pressure drop across the RO and the pressure drop due to friction loss in the piping. Both components are the same in both cases, it is only their order that changes. But as it is fundamentally a sum, the order of the summands should not matter.

The position of the RO should make no difference for the sizing of the RO or the actual minimum flow. At least if I understand the theory correctly, and I admit that I always struggle with that!

 

Could you please clarify?

 

kind regards,

Helmut

[fallah]

however, I still do not get the difference that the position of the restriction orifice makes.

fallah gives a good example (see quote) but it seems not complete.

 

Sengshel,

 

I did focus my explanation about the difference that the RO position makes just in OP's client comment standpoint as follows:

 

We have received a comment by our client asking us to move the orifices close to tank tie in in order to "avoid pressure build up in the recirculation line".

 

Not on difference in sizing the RO or the actual minimum flow...as you mentioned.

[HnAw]

Gita,

 

Refer "Scheme-7" sketch in the link below:

 

http://www.cheresour...trifugal-pumps/

 

Individual RO's in each pump discharge line should be provided if there are multiple pumps in paralel and an auto-start facility is provided.

 

If this is not the case (auto-start of multiple pumps in parallel) then a RO in the common discharge line can be provided which can be close to the tank where the liquid is being recycled. Providing the RO on the commond discharge line close to the pumps has certain disadvantages:

 

1. RO will need to be sized for a higher pressure drop if it is close to the pumps. Higher pressure drops can cause cavitation and noise.

 

2. A lower pipe rating downstream of the RO cannot be justified from a safeguarding point of view since RO's are subject to wear and tear. The normal practice is to rate the discharge piping including the forward flow piping and the recycle flow piping for the shut-off head of the pump.

 

3. If it is insisted that the pressure rating of the recycle line after the RO be reduced, then you may have to provide an additional safeguarding for the lower pressure rated piping downstream of the RO by providing a shut-down valve to close downstream of the RO in case the recycle line operating pressure exceeds the pressure rating of the piping. Additional expense of a shut-down valve and a pressure transmitter (diaphragm type) would be required. Alternatively a relief valve may be required in the recycle line to protect the piping. If a relief valve is installed it is also incurs extra cost and additionally the discharge of the reilef valve needs to be routed to a closed drain system  since we are talking about crude oil.

 

In fact I would recommend that you opt for "Scheme-6" provided in the link above if your minimum recycle flow is less than 30% of the pump design flow.

 

Hope this helps.

 

Regards,

Ankur..   

Dear Ankur,

in the case it has a high pressure drop requirement, with several orifices in line, what is the minimum distances between the plates?

Regards

Horacio

[breizh]

Horacio ,

 

Consider this resource ( multiple orifices).

hope this helps .

Breizh

Edited by breizh, 04 March 2015 - 08:09 PM.

[HnAw]

Breizh thanks for the info, the 4D distance is necesary even for water?

Edited by HnAw, 06 April 2015 - 07:38 PM.