CONTROL VALVES ON WELL/HEAD SEPARATOR SKIDS IN COAL SEAM GAS PLANTS

Transcription

1 CONTROL VALVES ON WELL/HEAD SEPARATOR SKIDS IN COAL SEAM GAS PLANTS HEAVY DUTY DESIGN FOR SEVERE SERVICE: CHOKE VALVE SOLUTIONS HIGH RANGEABILITY, SMOOTH CONTROL ACTION: CONTROL VALVE FOR GAS AND LIQUID HIGH RELIABILITY RMT Valvomeccanica srl Via dei Patrioti Solbiate Olona (VA) Italy Ph: Fax:

2 APPLICATION DESCRIPTION Technical Description Coal Seam Gas (CSG) is natural gas, consisting mainly of methane, trapped inside wide spread coal seams, which are areas of underground coal. Over millions of years, wide spread layers of decaying organic plant matter formed. Under the combination of temperature (earth s heat) and pressure (compression of upper earth layers) made possible the formation of these seams. They can be practically anywhere between 200 and 1000 meters below the earth s surface: natural gas is trapped in these layers, kept in position by the pressure of the water inside the coal seam and forming a thin film on its surface. Level of gas that can be produced from a coal deposit depends obviously on gas content, thickness of the coal, permeability and depth of the coal seam. A high potential CSG deposit is characterized by fractures in the coal bed that allow both gas and water to freely pass through its depth. Coal seam gas, are non-conventional plants. This is due to the way the CSG is brought to surface and extracted. To extract CSG the most efficiency way is to locate a great number of small wells spread all over the area of the deposit. In coal seam gas plants, we don t have one or few large well that extract several millions of barrel each day. Each well has generally the size of 3-4 inches, drilled into the ground until it reaches the coal seams. At the beginning, some water is extracted from the well, thus reducing the pressure in the underground: this make possible allowing the gas to flow and reach the earth surface. Sometimes the gas is not free to pass through the natural underground pathways. This depends on the quality of the site, in terms of permeability/depth/etc In this case the hydraulic fracking technique is used (more or less 30-40% of site need this fracking action). It consists in pumping a fluid mixture under pressure into the well. This fluid mixture is basically (99% and more) water and sand, plus a certain amount of salts and eventually eco-compatible chemical additives. The hydraulic pressure created by pumping the fluid into the well forces the fractures in the coal seams to open furtherly, creating a web of several gaps (few mm wide) that are fixed by the sands and formed preferential passages for gas flow to the surface. 2

3 Chemical additives are eco-compatibles and have the function to improve the fracking efficiency, facilitating sand suspension. Coal seam gas plants have a lower impact on environment than traditional ones: near this small wells land use (for instance farming) can continue and when depleted decommissioning operation are relatively easy and return the area to its natural environment. COAL SEAM GAS EXTRACTION After being brought to surface, the gas needs to be separated from water. After separation, water is then sent to a treatment facility while the gas is directed towards a processing plant, where it s treated, pressurized and prepared for transportation. Transportation is via long pipelines directly to downstream users or to an LNG (liquid natural gas) plants for liquefaction and export. In correspondence of wells usually there is the so called Well Head Separator skid (WHS)- Each skid is provided with a gas - liquid separator and some valves for its proper functioning. All these skids are spread into a wide area and need to be interconnected realizing literally a working network. For what concerns the control of the process, there are basically 3 main valves involved in a Well Head Separator Skid (WHS). - Liquid level control valve (LCV), that has to control the liquid level inside the separator and collect the fluid towards the water treatment facilities. - Gas pressure control valve (PCV), that has to collect the separated gas towards the gas processing plant. - Choke flow control valve (FCV CHOKE). This valve performs the first stage of pressure reduction, from the well pressure level to the operating pressure of the gas/liquid separator. Typical Well Head Skid for coal seam gas 3

4 APPLICATION ANALYSIS Due to the great amount of Well Head Skid, spread over wide areas of extraction, the use of remote operated control valves is practically mandatory. It a a good practice to use control valves rather than self-operated pressure reducing valve, make possible to inter-connect all the sites by a network. LCV valve controls the liquid level inside the separator. LCV faces therefore very LOW PRESSURE DROP. In order to maximize the separator capabilities Due to the different operating conditions of the well, this valve has to guarantee a HIGH RANGEABILITY. LCV deals with waste water, so great attention has to be put into material selection: a stainless steel trim and body should be preferred. Water could contain some coal fines, that represents a risk for SEATING RELIABILITY: also this shall be taken into consideration. In the separator the water level has to be kept inside a certain range, since usually the well head separators are not so big the LCV valve has to guarantee a very SMOOTH AND PRECISE CONTROL ACTION, joined together with a QUICK RESPONSE. PCV valve controls, at the outlet of the separator, the pressure of the gas being collected to the processing plants. Similarly to the LCV, also PCV operates at LOW DESIGN PRESSURE with SMALL PRESSURE DROPS, compatibly with separator operating conditions. Usually HIGH CV are required in order to be able to take full advantage of the process even at this relatively low working pressure. Since some coal fines could be present inside the gas flow, materials and treatment shall be carefully chosen for RELIABILITY. PCV shall be engineered with LOW FUGITIVE EMISSION and shall be capable of a QUICK and SMOOTH RESPONSE, in order to obtain an efficient control of the whole process. FCV CHOKE valve realizes the first stage of fluid pressure reduction, from the well head pressure down to the separator operating value. Obviosly FCV CHOKE it s the valve with the most severe service, it can suffer of some problems that can bring to well stop, usually it has to face up three main working conditions: well start up, well normal producing and well low production operational. - WELL START UP. Valve shall retain the well head pressure, that can rises up to 60-70Barg. Valve shall be capable to keep the closed position with high class of LEAKAGE for all lifetime service period. When it begins to open, high pressure drops occurs through the valve: from 60-70barg pressure has to be reduced to the 2-5 barg values which are the operating conditions of the separator. HIGH PRESSURE DROPS means very HIGH VELOCITIES inside the valve in the so called vena contracta. Moreover, it shall be noticed that the operating fluid is dirty: water, gas, coal fines and all that emerges from the well head has to pass through the valve during its modulation. There could be a serious risk of EROSION. - WELL NORMAL PRODUCING. During normal operation well pressure falls down to 6-20 barg, so that relatively low DP regards the valve- In this conditions valve shall work near to fully open position: in this phase high capacity (HIGH CV) are required. - WELL LOW PRODUCTION CONDITION. Choke valve shall be capable to handle quickly and with a stable response a turn from normal operational into a low production condition. This could occur for many reasons, such as a reduced demand or a change of extraction plan in the area. In this conditions FCV Choke shall close, thus reducing the flow rate. Pressure inside the well head comes again to rise together with high differential pressure conditions and related erosion risks. FCV Choke shall therefore offer HIGH RANGEABILITY: it shall be capable to pass from high CV and low DP conditions to reduced CV and high DP ones. Modulating working shall be characterized by SMOOTH CONTROL and QUICK POSITIONING. 4

5 RMT SOLUTIONS FOR WELL HEAD SKID IN CSG LCV solution by RMT is a V-CUT segmented ball control valve of 45 SERIES. Since low DP and HIGH RANGEABILITY are required, this valve is the right choice. Furthermore reduced face to face dimension is possible- Due the managed fluid, that is waste water, FULL 316 STAINLESS STELL DESIGN is provided (trim and body). Since coal fines particles could be suspended into the liquid, METAL TO METAL seating with HARDENED SEATING AREA is provided: 200µm of tungsten carbide coating (WCC) is the standard for this application. Leakage class up to V is possible. Similarly also PCV solution is a 45 SERIES V CUT segmented ball control (low DP and HIGH CV required). Standard materials are carbon steel for body and 316 stainless steel for trim. Coal seams fines could also be drag out with gas outside from the separator: METAL TO METAL design and seating surface hard coating PCV is designed with a LOW EMISSION PACKING, for fugitive emission in compliance with ISO class B. For both LCV and PCV LOW FRICTION DESIGN fo the seat and an accurate lapped surface of seat and ball are necessary, proper actuator selection helps in providing SMOOTH CONTROL AND QUICK RESPONSE. Depending on customer preferences, usually these valves can be equipped with: - Pneumatic actuator with gas-line supply. - Electrical actuator with fail safe action (if required) 45 SERIES V CUT BALL CONTROL vvvvvv 5

6 FCV CHOKE is the most critical valve in this application, characterized generally by a severe service. RMT proposes for this valve a special 45 series ball control valve which has been engineered with a specific design suitable to meet the requirements of the above-mentioned working condition. In this valve, plug element is a full-bore ball. On upstream side of the valve, there is a seating ring, while on the downstream side there is a V-SHAPED SPECIAL DIFFUSER. The upstream seat ring is the sealing element while the downstream V seat is the pressure reduction element. This special diffuser is responsible of the modulation and allows preserving the other trim components from erosion. V seat is made of solid tungsten carbide, with a hardness more than 1500 HV (EXTREME HARDNESS LEVEL) and it s special shape make it suitable to face up the most severe service conditions, that occurs when the valve has few degrees of opening and high pressure drops mean high velocities reached by fluid in the vena contracta. In this way, trim reliability is guaranteed and a LEAKAGE CLASS UP TO V can be reach and keep stable for all the service life of this special valve. Valve body can be of carbon steel or stainless steel, depending on the request and on the specific application. Trim is in 316 SS material (except the special V diffuser): ball and internal downstream body surfaces are hardened. The V diffuser gives the valve an engineered EQ% flow characteristic, suitable to meet the requirement of HIGH RANGEABILITY. Both low flow conditions and HIGH CV requirements are guaranteed with SMOOTH CONTROL AND QUICK RESPONSE, thanks to the LOW FRICTION DESIGN and careful actuator selection. Depending on customer preferences, usually these valves can be equipped with: - Pneumatic actuator with gas-line supply. - Electrical actuator with fail safe action (if required. Packing design allows meeting the LOW EMISSION requirements- Downstream SPECIAL V DIFFUSER 45 SERIES BALL CONTROL WITH SPECIAL NO EROSION TRIM 6

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