6 replies to this topic

[Travesh]

A spillback system in a hydrotreating plant experiences high noise levels and low reliability due to the letdown of pressure from approx. 126 kg/cm^2 g (pump discharge) to approx. 1.6 kg/cm^2 g at surge drum) at 38 degC. Please offer any modifications to improve the system. Details are described below. The MFD's for this system are attached. Are multistage angle let-down valves better or auto recirculation valves?

Feed pump J-15002 A spills-back to a surge drum, F-15001. The feed is a mixture of light cycle oil & light diesel (17.5 % light cycle oil, balance light diesel on volume basis) fed to a hydrotreater. The components of the spillback system include a control valve FV-15008, flow orifice and other valves.

Thank you.

Regards,
Travesh

Attached Files

•  Spillback_for_KOL.doc   128.5KB   82 downloads

[JoeWong]

For such high pressure drop, certainly it will experience cavitation and high noise level. Check with control valve vendor, control valve i.e. choke, angle, etc with low noise trim may helps.

[Qalander (Chem)]

For such high pressure drop, certainly it will experience cavitation and high noise level. Check with control valve vendor, control valve i.e. choke, angle, etc with low noise trim may helps.

Dear Travesh Hello/Good Morning, Seconding Joe Wong's concerns I will further add that localized High Velocity internal erosion issue could be sever at downstream of Spill Back valve and at joining location of recieving pipeline
Hope this helps
Best regards
Qalander

[sdarone]

Dear Travesh,
It is quite important the pressure drop you need to achieve, before the proposal of an alternative would you please provide some additional information on the control valve type, and requirements regarding flowrate control compared to the main stream.

The reason I'm asking is beacuse the spill back line is 4", whilst the main is 6", would it be possible to reduce the line size, to introduce additional frictional pressure drop, and reduce the requirement on the control valve?

Is the control valve single stage or multiple, the use of multiple stage control valve can probably improve reliability.
have you consider using a restriction orifice upstream of the control valve, this will increase the size of the control valve, but might help since the required pressure drop at the CV will be lower.

I think there are several alternatives, but need more info on the system and the restrictions.

Regards

Sergio

[Andrei]

Travesh,

There is a discrepancy between your post title, the explanation and the drawing. The title implies a pump minimum flow system, while the drawing shows a discharge flow control loop.
The minimum flow loops I know, usually are based on the pump discharge pressure, and are using more like on/off valves.
What you are showing in the drawing is using a modulating valve and is literally controlling the pump's discharge flow.
I do not understand why do you need a check valve on the spill-back line, when no reverse flow can occur because of the differential pressures in the system.
I suggest moving that flow orifice upstream of your control valve,
The distance from your control valve to the vessel should be minimum, actually the downstream isolation valve should be directly on the vessel nozzle and the spool to you control valve should be very short, actually an extension.
I do not see an extension downstream of the valve to handle the flashed fluid; your line diameter is the same all the way from pump discharge line to the vessel.
You should study the possibility of eliminating the downstream isolation valve; why do you need it; do you really need it? This way you can move your control valve even closer to the vessel.
The main idea is: minimize the length of the two-phase line, downstream of your valve. And size it properly of course.
And another thing: your main discharge process is 6" diameter while the spill-back is 4". That is probably allowing more than 2/3 pump flow to be recycled. Normally a pump minimum flow is around 35% of it's rated flow. You can say that you have a restriction orifice, but your lines diameters should reflect this kind of proportion.
Good Luck

[fallah]

while the drawing shows a discharge flow control loop.
The minimum flow loops I know, usually are based on the pump discharge pressure, and are using more like on/off valves.
What you are showing in the drawing is using a modulating valve and is literally controlling the pump's discharge flow.

Configuration for minimum flow is correct as FV 15008 would be opened via FIC sensing minimum flow at predefined set point in discharge line,but locating FO downstream of FV may cause not having smooth minimum flow (and even may reduce desired flow) and would not necessary in minimum flow line.
If it is attended to let down the pressure in spill back flow line (as per mentioned in first post), it would be better using PCV controlling pressure in its downstream.

[djack77494]

Configuration for minimum flow is correct as FV 15008 would be opened via FIC sensing minimum flow at predefined set point in discharge line,but locating FO downstream of FV may cause not having smooth minimum flow (and even may reduce desired flow) and would not necessary in minimum flow line.

I concur and believe you have a very typical and acceptable configuration for your design. There are two items I would consider changing. First, I do not like to install a restriction orifice in series with a control valve as it serves very little purpose AND it tends to reduce the ability of the control system to actually control your process. Second, I concur with Andrei's viewpoint that the control valve can be located near the surge vessel's nozzle with potential resulting savings of eliminating the valving downstream of the control valve. I like the direct approach to controlling the system that is depicted here. You want to control the minimum flow through the pump. So, you measure the flow through the pump and directly control it to achieve at least the desired minimum. There is no reason to use a "backhanded" control scheme such as measuring and controlling pressure. Why not be direct?