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| Home | Separate
your Separators
We know them as separators, filter separators, Knockout pots, Cyclone Separators, Centrifugal separators, Vertical Separators, SCUDs, Horizontal Separators, Spherical Separators, Flash tanks, Scrubbers, Slug Catchers, Line Drips, and I’m leaving out a few more. All of the above vessels are designed to do what their name indicates; separate hydrocarbon streams produced at the wellhead into their constituent phases. This can mean separating gas from water, gas from oil, water from oil, and, in the case of a three phase separator, water from gas from oil. The principles behind separators are simple. Momentum, gravity settling, and coalescing are used to separate the phases. One, or more of these principles can be used in a separator. Momentum can be used to separate liquids by causing an emulsion stream to suddenly change direction. The heavier part of the emulsion cannot change direction as quickly as the lighter part can, and thus separation occurs. Gravity can also be of great help. Ask the good folks at Exxon which is heavier, oil or water, and they will show you a beach where some oil floated ashore. Note that this is not always the case as oil of API gravity around 11 starts to become heavier than water and will actually reside underneath water in a vessel. This is often called phase inversion, and keeps engineers very occupied. With the help of C. Richard Sivalls’ "Oil and Gas Separation Design Manual", we can identify some common features on separators of all kinds:
Design criteria differ a lot between a low pressure oil well, (made up mostly of a large volume liquid phase) and a high pressure gas well, where the gas volume is high, and the liquid is often a high gravity hydrocarbon. Either one may contain free water as well. Some other factors affecting separation are operating pressure, temperature, and composition. Generally speaking, an increase in pressure, or a decrease in temperature will positively affect separation. Most vessels have internal baffles in place. The less expensive separators are the ones you get when you walk into a fab shop and tell them you want a 16" separator. Unless you specify more, this may be a can that is rated for the pressure you need and has all the outlets and inlets a separator should but not much more. If the manufacturer has more on his mind than the bottom dollar, and wants you back, you’re o.k. But not every person with a torch and a garage knows separators. What separates a separator that can capably handle 3 phases from an "off the rack" 3 phase separator dressed for 2 phase operation is the internals. In order for a separator to be able to capably deal with the liquids within it, a number of baffles and shields are required. These provide the floats with a quiet chamber that’s not constantly turbulent. The baffles also separate the phases from the incoming emulsion so that it cannot be constantly "upset" every time a slug of fluid/gas comes in. It is quite common to see low dollar separators with a float in the wide open diameter of the vessel with the only thing protecting it is a little square piece of steel somewhere 2 feet above it. These may work in some instances, but if you really want to know how much water and oil you have before you meter it, think about buying a brand-name separator. Another often overlooked factor in the purchase of a separator is the mist extractor. This mist extractor is made of stainless steel woven wire and is proven to remove 99.9% of the entrained liquids from the gas stream. Placement of this mist extractor is crucial to its performance. Joe Buril, an experienced designer with Grand Valley Design notes that poor placement of such a pad in a horizontal separator is a great source of trouble for producers. What happens is that due to poor designing of an original vessel, a lot of liquids get carried over from the there into the gas line. If instead the customer would have spent a bit more time (Note I’m not saying more money) looking over the design of the vessel, they could have saved themselves the downstream hassles of entrained liquids. (and of course the extra vessel you now need to separate these liquids) Once you’ve found a manufacturer you’re comfortable with consider the configuration, horizontal or vertical. Verticals are usually chosen when there’s more gas than liquid. If there are liquid slugs anticipated , but overall low liquid rates are likely, an simple increase in the height will keep you covered. Horizontal separators can handle larger volumes of liquids, and also will help release dissolved gas from the liquids. The larger interface area makes a big difference here. Some variations on these themes are "boots". Picture a horizontal vessel that may occasionally see a gas or water slug. By adding a small vertical portion to the vessel you combine some of the advantages of each configuration and buy some residence time to boot. (sorry about the pun) Odd balls are the spherical separators. (Sorry I couldn’t help myself here) These separators provide more economical alternatives to people with very high-pressure situations. The sphere provides a maximum of protection with a minimum of steel. There are lots more designs out there, amongst which are the very small separators that can often be seen in shallow gas areas. Here, small amounts of liquids and gases need to be separated prior to measurement, but on an economical basis. These units are becoming more common, and are being sold for prices under $15,000.00 including all the bells and whistles expected on a larger skid such as catalytic heat, meters, PSV’s, Fuel Gas Scrubbers etc., and all that in a 4’ x 5’ box hanging off a small diameter pipeline. The interesting thing is that not all separators are alike, and that 2 units that are described the same way on a quotation can look drastically different on the draftsman’s table; so keep ‘em separate! |
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