Crude Oil Quality

[jmalter]

I'm working with a control system for a crude oil pipeline and I'm interested in the amount of particles in the oil. We will be recirculating the oil around the pumps thorugh a control valve and therefore will have a high pressure drop (1440 psi discharge, 35 psi suction). Some of the control valves have trims with small holes to reduce the cativation. I'm concerned if there are particles in the crude it will clog the control valve or a strainer/filter.

Is it common for crude to have particles? Are some crudes cleaner then others?

Joe Malter
Pipeline Systems Inc.

 

[TD2K]

Crudes can definitely have solids, more so as you move upstream to the wellhead. I know of one oil separation plant that had minimum flow valves with a pressure drop comparable to yours that also originally installed cavitation control trim.

They made great filters.

 

[jmalter]

Is it practcal to think we can add strainers to keep the control valve clean (and not have to clan the strainer every hour)?

BTW this is purchased crude off a tanker ship. I would think there is some kind of spec for allowable suspended solids and particle size.

 

[TD2K]

It's all a matter of the particle size the valve will allow to pass through the trim (talk to the vendor) versus what is in the crude. That will in turn set the load on the strainer so you can select a strainer with sufficient capacity it doesn't have to be cleaned hourly.

That said, my experience was with a field processing plant just downstream of the wells where you expect to have sand and other solids. If your crude is being off-loaded from a ship, it's been processed (stabilized) for vapor pressure at the very least and most of the bigger solids will, in my opinion, be gone.

Crudes do have BS&W specs which would limit the amount of solids and water but I'm not familar with max particulate limits being part of that spec. That said, typical crude limits are 0.5% BS&W or less which means only the finest particles are going to be left in the crude (a good portion of the BS&W is water) which should go through your valve. I would check with my valve supplier for the trim hole opening and see if you still think you might have a problem. Ask for similar references and talk to them.

 

[jmalter]

Thanks for the reply. We purchased a control valve with an anit-cavition trim. The holes are 0.05". We only found out the size after purchasing and after having problems. No where in the documentation does it indicate filtering or hole size.

We installed a filter with a 16 mesh but still had problems. Now we are looking at using a finer mesh (30) but suspect the filter will clog rapidly. We think our vendor supplied the wrong type valve/trim to make a sale. Our data sheet indicated the product as crude.

There were (and are) strainers on the suction side of the pumps but with a larger mesh. We purchased a 600 class strainer and installed right before the control valve. The particles are smaller then the cavitrol holes but we suspect that multiple particles are trying to go through the hole at the same time.

I'm trying to get references out of our vendor so I can get some help resolving this problem.

 

[TD2K]

The solution at the plant I mentioned was to install restriction orifices (ROs) downstream of the valve and convert the valve to a standard trim.

The theory was to size the ROs to take enough pressure drop so that you didn't need cavitrol trim on the control valves. The problems is of course that the dP across the ROs is flow dependent. As you reduce flow through the recycle valve, the dP through the ROs decrease and the dP the valve has to take increases and at some point, the control valve starts to experience unacceptable cavitation. At what point is the basis you size the ROs for.

I'd talk to the valve vendor and determine how much backpressure you need to go with a standard trim and then see if an RO might work for you.

 

[ICman]

This looks like you want two exclusive things from your valve - ability to handle dirty fluids AND noise / cavitation reduction (at a high pressure drop, no less).

TD2K's comments on using ROs are good. You crude is still dirty, but you may not need the Cavitrol trim.

You could also add a valve in series, to take a "double-cut" on the pressure. I don't know if it would help, since I don't know the specifics of your application.

Crude from a tanker is usually fairly clean. After all, the pump has to handle those particles, too. You might want to look at a different type of filter (or a strainer), one that allows the particles to drop out of the flow stream, to avoid clogging.