22 replies to this topic

[go-fish]

Could someone please explain the purpose of the pressure control valve in a gasoline blending manifold? One of the components in the blending is n-butane. I believe it has something to do with preventing butane from flashing but I am not able to completely comprehend how the valves help in preventing that.

Thanks

[ankur2061]

Could someone please explain the purpose of the pressure control valve in a gasoline blending manifold? One of the components in the blending is n-butane. I believe it has something to do with preventing butane from flashing but I am not able to completely comprehend how the valves help in preventing that.

Thanks

go-fish,

I believe you must be having a system configuration as a sketch or flow diagram for the gasoline blending manifold. Considering that you have a flow diagram for your system, you are not able to comprehend how the PCV will work then imagine the plight of the readers of the post who simply don't know how your system looks like & want to help you.

Can you post a sketch please.

Regards,
Ankur.

[Himanshu Sharma]

Sir,
infact more information is needed thus answer will be having a 'may be'

Probably secondary function of this valve is to control flow rate of Butane thereby serving the primary purpose controlling RVP and hence meeting the spec in an online blending scenario.

[go-fish]

Please refer the attached sketch. I have red marked the control valve under discussion.

Attached Files

•  BLENDING SKETCH.pdf   16.96KB   92 downloads

Edited by go-fish, 12 April 2010 - 01:43 PM.

[demank]

Please refer the attached sketch. I have red marked the control valve under discussion.

Hallo,
According to your sketch,
That PCV installed is to make sure that the inlet mixer in the condition of constant pressure (or desirable pressure).
Without PCV, the inlet mixer pressure will change depend on the source...,

[ankur2061]

go-fish,

Yes, your thinking is right. Vapor pressure of n-Butane is 3.8 bar (abs) @40°C. If the operating temperature at the mixing manifold is in that range, you will certainly need to maintain a minimum back-pressure to prevent flashing and also for the proper working of the inline static mixer. Static mixers definitely need a certain amount of back-pressure for efficient mixing.

Hope this helps.

Regards,
Ankur.

[fallah]

Could someone please explain the purpose of the pressure control valve in a gasoline blending manifold? One of the components in the blending is n-butane. I believe it has something to do with preventing butane from flashing but I am not able to completely comprehend how the valves help in preventing that.

Thanks

Blending of some liquid components with different vapor pressures in liquid phase need to maintain the pressure well above the corresponding vapor pressure of the most volatile component at operating temperature.

Edited by fallah, 13 April 2010 - 02:47 AM.

[go-fish]

go-fish,

Yes, your thinking is right. Vapor pressure of n-Butane is 3.8 bar (abs) @40°C. If the operating temperature at the mixing manifold is in that range, you will certainly need to maintain a minimum back-pressure to prevent flashing and also for the proper working of the inline static mixer. Static mixers definitely need a certain amount of back-pressure for efficient mixing.

Hope this helps.

Regards,
Ankur.

How does PCV located downstream of inline mixer help in maintaining this minimum back pressure? Could you please explain in detail or provide some reference where I can get hold of these basics. I am comparatively new to Process Systems engineering.

[ankur2061]

go-fish,

Yes, your thinking is right. Vapor pressure of n-Butane is 3.8 bar (abs) @40°C. If the operating temperature at the mixing manifold is in that range, you will certainly need to maintain a minimum back-pressure to prevent flashing and also for the proper working of the inline static mixer. Static mixers definitely need a certain amount of back-pressure for efficient mixing.

Hope this helps.

Regards,
Ankur.

How does PCV located downstream of inline mixer help in maintaining this minimum back pressure? Could you please explain in detail or provide some reference where I can get hold of these basics. I am comparatively new to Process Systems engineering.

go-fish,

The PCV in your sketch is a back-pressure control valve. Essentially, it means that the line pressure upstream of the control valve is sensed by the sensing element (transducer). The sensed pressure is then compared with the setpoint of the pressure controller. In case the measured pressure is less than the set-point the control valve action is to close till the measured pressure matches with the setpoint. The control valve action is to open when your measured variable is more than the set-point till it again matches with the set-point.

The set-point for the back-pressure PCV is fixed based on a suitable margin above the vapor pressure of the mixed liquid stream upstream of the control valve.

I hope this explanation is good enough for you to understand.

Regards,
Ankur.

[go-fish]

go-fish,

Yes, your thinking is right. Vapor pressure of n-Butane is 3.8 bar (abs) @40°C. If the operating temperature at the mixing manifold is in that range, you will certainly need to maintain a minimum back-pressure to prevent flashing and also for the proper working of the inline static mixer. Static mixers definitely need a certain amount of back-pressure for efficient mixing.

Hope this helps.

Regards,
Ankur.

How does PCV located downstream of inline mixer help in maintaining this minimum back pressure? Could you please explain in detail or provide some reference where I can get hold of these basics. I am comparatively new to Process Systems engineering.

go-fish,

The PCV in your sketch is a back-pressure control valve. Essentially, it means that the line pressure upstream of the control valve is sensed by the sensing element (transducer). The sensed pressure is then compared with the setpoint of the pressure controller. In case the measured pressure is less than the set-point the control valve action is to close till the measured pressure matches with the setpoint. The control valve action is to open when your measured variable is more than the set-point till it again matches with the set-point.

The set-point for the back-pressure PCV is fixed based on a suitable margin above the vapor pressure of the mixed liquid stream upstream of the control valve.

I hope this explanation is good enough for you to understand.

Regards,
Ankur.

Thanks for the explanation.

Is this PCV always required?

In my case, n-butane is pumped from the sphere to the blending manifold and the blended gasoline product is transferred to an atmospheric tank. The line hydraulics were performed by keeping end point pressure (atmospheric tank) as 0 barg. From hydraulics, it is found that the operating pressure always stays well above n-butane TVP upto the downstream end of the inline mixer, do we still require this PSV?

[kkala]

Vapor pressure of n-Butane is 3.8 bar (abs) @40°C. If the operating temperature at the mixing manifold is in that range, you will certainly need to maintain a minimum back-pressure to prevent flashing and also for the proper working of the inline static mixer. Static mixers definitely need a certain amount of back-pressure for efficient mixing.

I would say that static mixers require some min ΔP (so some min flow), plus no flashing at the end, the latter being realized by a downstream control valve (shown at the attached sketch by go-fish). Nevertheless such PCV is also understood to improve stability of control, at least according to interpretation below.
A local PID of existing gasoline (static) mixer has been looked into; control is same as in the sketch; pressure downstream mixer is kept at 4.5 kg/cm2 g through a PCV. There is a flow meter between mixer and PCV, so there is a reason to eliminate possibility of flashing upstream PCV.
But looking at a PID of an existing fuel oil mixer (no possibility of flashing), same controls have been found, including the downstream PCV. This PCV can aim at a more reliable control, in combination with FCVs.
For instance, suppose that one FCV suddenly closes a bit, reducing flow. If there is PCV (case 1), it will also close to keep its upstream pressure constant (pressure control action is fast). Then FCV will gradually open again, followed by PCV (same action).
If there is no PCV (case 2), pressure downstream mixer will be reduced due to lower flow (to storage); this will tend to increase flow through FCV even before FCV starts opening again; when FCV gets normal opening, flow may exceed normal, needing corrective action (2nd order system response).
So case 1 looks more stable than case 2. Nevertheless these are just thoughts. Mathematical treatment could have a certain result.
At any case pressure control downstream of a static mixer seems to be a recommended practice (even when there may be no flashing).

[demank]

Is this PCV always required?

From hydraulics, it is found that the operating pressure always stays well above n-butane TVP upto the downstream end of the inline mixer, do we still require this PSV?

Hydraulic calculation is calculated with ideal condition. How about when inlet pressure of each source is changed accidentally? How about during startup and shutdown?
As my previously posted. This PCV is need, to maintain the pressure of the mixer.
If the mixer pressure is unmaintained, pressure change (below vapor pressure) probability become high.

[fallah]

The PCV in your sketch is a back-pressure control valve. Essentially, it means that the line pressure upstream of the control valve is sensed by the sensing element (transducer).

Just minor comment on above:

The PV in the sketch isn't a back pressure control valve,because a back pressure control valve opens upon loss of downstream pressure.It is a pressure control valve would operate upon its upstream pressure.

[fallah]

it is found that the operating pressure always stays well above n-butane TVP upto the downstream end of the inline mixer, do we still require this PSV?

What about the pressure loss between end of the inline mixer and exit of the line after mixer (entrance of flow to storage)?

Do you certain between above two points the flow wouldn't become two phase flow?

[go-fish]

it is found that the operating pressure always stays well above n-butane TVP upto the downstream end of the inline mixer, do we still require this PSV?

What about the pressure loss between end of the inline mixer and exit of the line after mixer (entrance of flow to storage)?

Do you certain between above two points the flow wouldn't become two phase flow?

After mixing, since n-butane will only be a small percentage in the blended gasoline product. The TVP of the gasoline is much lower than that of pure n-butane.

[kkala]

But looking at a PID of an existing fuel oil mixer (no possibility of flashing), same controls have been found, including the downstream PCV. This PCV can aim at a more reliable control, in combination with FCVs.
For instance, suppose that one FCV suddenly closes a bit, reducing flow. If there is PCV (case 1), it will also close to keep its upstream pressure constant (pressure control action is fast). Then FCV will gradually open again, followed by PCV (same action).
If there is no PCV (case 2), pressure downstream mixer will be reduced due to lower flow (to storage); this will tend to increase flow through FCV even before FCV starts opening again; when FCV gets normal opening, flow may exceed normal, needing corrective action (2nd order system response).
So case 1 looks more stable than case 2. Nevertheless these are just thoughts. Mathematical treatment could have a certain result.
At any case pressure control downstream of a static mixer seems to be a recommended practice (even when there may be no flashing).

Further to the above I have looked at a local PID of an existing diesel oil static mixer. Controls are same, with PCV downstream of mixer.
So gasolin, diesel, fuel oil static mixers (blenders) of same refinery have FCVs on the feeding lines and PCV to keep contant pressure downstream of the mixer.

Edited by kkala, 14 April 2010 - 12:44 PM.

[kkala]

What about the pressure loss between end of the inline mixer and exit of the line after mixer (entrance of flow to storage)?
Do you certain between above two points the flow wouldn't become two phase flow?

You mean that flow may get two phase downstream PCV on its way to storage. Yes, this can occur, if mentioned PCV aims at no flashing upstream of it (PCV can have another purpose indicated in previous post). Nevertheless usually a very small percentage of liquid is flashed (some n-C4H10 in mentioned case). This could deteriorate flow meter reading (probably efficient mixing too), if it had occurred
upstream PCV (in the PIDs seen there is flow meter between mixer and PCV). But it is usually unimportant downstream PCV, where there are no flow meters.

[fallah]

After mixing, since n-butane will only be a small percentage in the blended gasoline product. The TVP of the gasoline is much lower than that of pure n-butane.

Any way,if you don't want to consider PV in mixer downstream and also want to maintain a pressure well higher than vapor pressure of the n-Butane (at moderate operating temperature almost 4 bara) along the mixer,you should adjust the pressure profile such that you have delta P=almost 3 bar across end of the mixer and fluid outlet (entrance of storage) that may provided by lengthening the line after mixer (may be almost 3 km considering 1 bar pressure drop per 1 km) or using a RO or.....

[fallah]

What about the pressure loss between end of the inline mixer and exit of the line after mixer (entrance of flow to storage)?
Do you certain between above two points the flow wouldn't become two phase flow?

You mean that flow may get two phase downstream PCV on its way to storage. Yes, this can occur, if mentioned PCV aims at no flashing upstream of it (PCV can have another purpose indicated in previous post). Nevertheless usually a very small percentage of liquid is flashed (some n-C4H10 in mentioned case). This could deteriorate flow meter reading (probably efficient mixing too), if it had occurred
upstream PCV (in the PIDs seen there is flow meter between mixer and PCV). But it is usually unimportant downstream PCV, where there are no flow meters.

As you note to previous posts,i mean situation without PV in downstream.......

[kkala]

It is noted that flashing is not expected to happen under normal operating conditions. The fluid will not be designed for flashing along its way to atmospheric storage, which can cause a lot of problems there.
Flashing can occur under extreme conditions, e.g. if highly volatile material (say propane) is wrongly mixed, or if proportion of n-butane gets extremely high after some loss of control. For same wrong composition of gasoline, in case of PCV or without it, accomplished flashing will be same at storage tank (where pressure is 0 Barg). PCV seems not to reduce total flashing, but gives some certainty of no flashing upstream of it.

Edited by kkala, 18 April 2010 - 11:45 AM.

[CPCE]

Could someone please explain the purpose of the pressure control valve in a gasoline blending manifold? One of the components in the blending is n-butane. I believe it has something to do with preventing butane from flashing but I am not able to completely comprehend how the valves help in preventing that.

Thanks

We have a slightly different situation to tackle for a new project - it is necessary to have a booster pump downstream of the pressure control valve, after the blending manifold and in line static mixer. The PCV is thus in the suction line of the booster pump. The concern is for the safety of the pump due accidental suction flow loss due to the closure of PCV. Immediate thought is to shift the PCV to the discharge side of the pump, retaining the pressure sensor on the suction side. Will this Pressure control loop work? Any ideas?

[kkala]

We have a slightly different situation to tackle for a new project - it is necessary to have a booster pump downstream of the pressure control valve, after the blending manifold and in line static mixer. The PCV is thus in the suction line of the booster pump. The concern is for the safety of the pump due accidental suction flow loss due to the closure of PCV. Immediate thought is to shift the PCV to the discharge side of the pump, retaining the pressure sensor on the suction side. Will this Pressure control loop work? Any ideas?

PCV shift to the discharge of "booster" pump looks justified to avoid extra ΔP in its suction line, which could create additional flashing (under not normal conditions).
PI (sensor) on mentioned suction would close PCV on detecting low pressure; reducing "booster" pump flow rate, thus NPSH required; and at same time increasing suction pressure (assumed to be the discharge of another pump).
I am not familiar with this control scheme, though it seems correct. Calibration of PI - valve closure may be difficult (stability?). Of course the PCV would be failed open (FO) and "booster" pump should have minimum flow line upstream PCV.
Another thought is to shift PI to the "booster" pump discharge too, but then a more complicated control would be required with a FI also participating.
A similar thread (Question about booster pump control) was posted by jangway on 6 Apr 10 (answer 11 Apr 10).
Please inform us on the development / results concerning this issue.

[CPCE]

We have a slightly different situation to tackle for a new project - it is necessary to have a booster pump downstream of the pressure control valve, after the blending manifold and in line static mixer. The PCV is thus in the suction line of the booster pump. The concern is for the safety of the pump due accidental suction flow loss due to the closure of PCV. Immediate thought is to shift the PCV to the discharge side of the pump, retaining the pressure sensor on the suction side. Will this Pressure control loop work? Any ideas?

PCV shift to the discharge of "booster" pump looks justified to avoid extra ΔP in its suction line, which could create additional flashing (under not normal conditions).
PI (sensor) on mentioned suction would close PCV on detecting low pressure; reducing "booster" pump flow rate, thus NPSH required; and at same time increasing suction pressure (assumed to be the discharge of another pump).
I am not familiar with this control scheme, though it seems correct. Calibration of PI - valve closure may be difficult (stability?). Of course the PCV would be failed open (FO) and "booster" pump should have minimum flow line upstream PCV.
Another thought is to shift PI to the "booster" pump discharge too, but then a more complicated control would be required with a FI also participating.
A similar thread (Question about booster pump control) was posted by jangway on 6 Apr 10 (answer 11 Apr 10).
Please inform us on the development / results concerning this issue.

Thanks very much for your prompt response. Will be finalising the scheme as posted before. Will certainly post outocome.