5 replies to this topic

[AH1979]

I have a problem I am trying to find a solution for, the background is as follows.

Approximately 4 MMBPD of oil is flowing to a crude stabilization facility via a pipeline. This unstabilized crude oil is then fed to a number of spheroid separators where the oil and gas is separated. Any water in the crude oil settles in the spheroids where it is drained. During normal operation this poses no problems. However, sometimes a large slug of water will come through with the crude, this may occur on a daily basis and during pigging operations the volume of the slug of water can be large enough that it overwhelms the entire facility. I am looking at options for removing these large slugs of water that occur from time to time before they get to the spheroids. I have been considering some kind of three phase separator upstream of the spheroids but I am worried that with there being a significant amount of gas in the oil, this would overwhelm any new three phase separator since the gas/liquid separation load would simply shift from the spheroids to the separator.

Wondering if anyone has any good ideas.

Edited by AH1979, 23 January 2011 - 01:53 AM.

[Zauberberg]

Based on my experience - and substantiated with several field installations - you need a liquid dump vessel downstream of a 3-phase separator (slug catcher), where any excess liquid, predominantly water, will be dumped during transient operation. Normally, when you get back into steady-state mode, a controlled flow of liquid stream from the dump vessel is routed to the liquid discharge line from the phase separator, and you will have a controlled gravity separation inside the spheres. Slowly by time, you can process the whole inventory of the dump vessel without any process upsets.

[kkala]

Slug catcher (according to Zauberberg's post) seems to be the solution; an additionally measure we had thought of (but not realized) during slops upgrade design may be useful. The problem was that water chunks from slops tank went directly into vacuum distillation column, disturbing its normal operation for (say) 8 hours. This apparently occurred when settling water level in the slops tank approached pump suction.
α) We first thought of an interface level indicator (water/slops) to prevent high water level in the slops tank (crude tank or even spheroids in the reference case); this did not proceed, even though we had some indications that such instrumentation exists in the market. Actually no similar installation existed in the refinery; and the fact that a heavier than water hydrocarbon phase eventually existed (settling below water level) made things complicated.
β)Then we thought of an online conductivity meter to warn operator about the incoming chunks of water (there was possibility to stop slops feed into the column and drain slops tank and line - for the crude case it may not be so, but even a warning is valuable). Nevertheless Client canceled the project at that time, so nothing was implemented.
The reaction of Operations indicated that water chunks was a serious problem. No thought of slug catcher was made at that time (a more radical solution).

[Art Montemayor]

As I understand it, the basic problem in your present operation is one of a successful 3-phase separation with a very large crude oil flow rate (176.7 m3/sec !!!) that can include slugs (or pockets) of water.

I am assuming the water originates at the field production facilities – that are probably feeding a main crude oil “trunkline” pipeline that collects all production and transports it to your processing facilities. Any lapse or bad crude oil production operation at the wellheads (no heater-treaters, no 3-phase separators, no desalters) means that any wellhead production unit can inject a “pocket” of produced water into the truckline and cause the apparent upsets downstream at your crude reception facilities.

The extremely large crude oil flow rate explains the presence of spherical vessels instead of conventional cylindrical ones. However, the use of spherical vessels in this service immediately brings up the following negative characteristics of such vessels:

• The inherent geometric shape of a sphere makes it very difficult to apply it as a phase separator because the superficial velocities within the vessel are constantly varying with the liquid level variances. The best (slowest) vapor velocity is at the mid-level point – but this drastically reduces the total capacity of the vessel. Unlike a horizontal, cylindrical vessel, it can’t reduce the superficial velocity by extending its length. Hence, the crude oil liquid level has to be carried at a relatively higher position – probably close to ¾ of the diameter. This makes the contents very sensitive to any potential inlet flow upset due to the fact that any liquid entrainment cannot be tolerated in the vessel if it is also separating a large amount of associated gas.
• The sphere can’t have a weir installed internally to serve as compartmentalized separator because of the large reduction in volumes in each compartment that can’t be modified by extending length – as is done in a cylindrical vessel. This means you must rely on the natural decantation that takes place inside the sphere due to natural settling – which is set by the residence time and the temperature. The only way to compensate for -and handle – any water slugs fed into the sphere, you probably are setting the water-crude oil phase level rather low in the vessel. But that is an area where any change in level is rather “fast” due to the varying top diameter of the water pool at the bottom. Spherical vessel are great and economical storage vessels when applied to large inventories. However, they are a pain in the back side when applied to process operations. All of their positive attributes seem to fall apart when they are introduced into processing operations and are expected to function as something else besides a static storage vessel. I am sure your engineering department and company are well aware of this and these features were taken into consideration when spheres were selected to serve as 3-phase separators.

I don’t know if my ideas are good for your operation but knowing only what you have told us, I would concentrate on eliminating any water slugs being fed into the spherical separators. In order to do this, I would design and install upstream separation equipment meant to serve as capacitance vessel(s) in order to ensure that the feed to the downstream spheres is as even, homogeneous, and steady as possible. Unfortunately, if your trunkline is operating with mixed flows (associated gas + crude + produced water) then you have a more serious problem – that the new upstream vessels be also 2-phase separators (gas + liquids). This increases the size and related gas handling requirements. This type of scope goes beyond just "cathching a slug"; it involves phase separation and handling.

This is not going to be a low-cost modification – due to the tremendous crude handling capacity you have to confront. But the cost of controlling or shutting down the downstream crude handling and processing equipment will determine just how important the solution is and how much economic gain there is to making a modification.

[Zauberberg]

See attached a DCS snapshot of inlet reception facilities showing both the slug catcher (inlet separator, X-3102), and the liquid dump vessel, V-3151. You can also see some of the basic control loops provided to ensure rapid dumping of liquid/water slugs into V-3151, and subsequent controlled flow to the main liquid/condensate discharge from X-3102. This arrangement also manages any gas flashed from the V-3151 dump vessel.

The dump vessel should be sized based on maximum anticipated slug volume, either during startup, pigging, or any other activity that may result in large amounts of liquid phase entering the plant/X-3102.

Attached Files

•  Inlet Reception.jpg   118.23KB   20 downloads

[AH1979]

Thanks for you responses.

Art Montemayor, your flowrate is a bit high, more like about 7 m3/s but you are correct it is a very high flow.

Also regarding the spheroids. We are talking about slugs of water in oil. Since these are both liquid phases, it may not have that much impact on the overall liquid level and gas/liquid separation. The main issue is that if a large amount of water comes through, the current system is unable to dewater the crude effectively and water ends up being carried over with the crude.