Controlled and Non-controlled Type Depressuring. Oil and gas facilities is generally required to be depressured to a safe level within a reasonable time during emergency situation. The requirements are clearly stated in the API Std 521 2007 edition, section 5.20.1. In earlier post "How to apply valve equation in HYSYS Depressuring ?" where i have discussed what, where and how to apply vapor flow equations in the HYSYS Depressuring valve parameter page. In that posts, i have mentioned about controlled and non-controlled type depressuring but without detailing them. In this i will elaborate a bit more the function and application of controlled and non-controlled type depressuring.
Non-controlled Type Depressuring Non-controlled type is the most ordinary type of depressuring arrangement. Generally it consists of a Blowdown Valve (BDV) for isolation purpose with a correctly sized restriction orifice (RO) to limit peak depressuring rate.
Non-controlled type depressuring is commonly apply throughout plant depressuring system. It can depressure the system from initial pressure (Pi) to final pressure (Pf) within required time frame (i.e. 15 minutes). As it is a fixed bore restriction orifice, the initial depressuring rate is high and gradually decrease to minimum depressuring rate at the final condition. This arrangement is conventional, simple and reliable for most depressuring system without any process or equipment limitation.
To model this type of depressuring in HYSYS rather simple, i would always propose to use the [General] vapor flow equation for critical flow case and [Subsonic] vapor flow equation for subcritical case.
Controlled Type Depressuring For some systems such as compressor and mole sieve bed, the depressuring rate needs to be controlled in order to avoid damage of compressor seal and mole sieve bed. This requirement generally imposed by the equipment supplier to the client and they generally limit the rate of pressure drop e.g 20 bar / min for compressor seal, 50 psi/ min for mole sieve bed, etc. With this additional requirement, an ordinary noncontrolled type depressuring (single BDV+RO) may not meet both requirements. It could either meeting the 15 minutes depressuring time but with high rate of pressure drop during initial depressuring or meeting the rate of pressure drop but with extended depressuring time (>15 minutes). A controlled type depressuring method is required. First type would be multiple BDV with RO (smaller). See below image.
The idea is to provide small opening for depressuring during initial depressuring, as the system pressure is reduced, the opening for depressuring is gradually rise to increase the depressuring rate. Opening of BDVs will be staggered according to time in order to limit the rate of pressure drop whilst depressuring the system pressure within the required time (i.e. 15 minutes). The method is step change of opening and depressuring rate will change from time to time but within a flow rate band. Second type would be a flow control valve with a flow control loop. See below image.
It basically to maintain a constant depressuring rate by varying the valve opening. It can limit the rate of pressure drop whilst depressuring the system pressure within the required time (i.e. 15 minutes). It can maintain a rather constant flow throughout the entire depressuring period, however reliability could be a major issue in most application. To model the first type of controlled depressuring using multiple BDV & RO in HYSYS is rather complicated. Presently (version 2006) there is no multiple depressuring unit is available in HYSYS. I would propose to conduct series of batch depressuring (similar to single BDV+RO case) with different orifice size. To model the second type of controlled depressuring using FCV, it is proposed to use the vapor flow equation of [Fisher] and [Masoneilan] may be applied. This involve trial-and-error to find a suitable control valve module to facilitate the depressuring rate.
How to apply valve equation in HYSYS Depressuring ? High pressure and moderate/low temperature operation are common in oil and gas industry. High pressure system with large vessels will lead to large inventory and it significantly increases hazardous level to personnel and assets. Thus, API Std 521 has stated clearly and requested plant emergency response system to depressurize the system from it maximum operating pressure to 50% of it design pressure or 6.9 barg, whichever is lower within 15 minutes. The intention is to evacuate the inventory within an acceptable time limit to minimize risk of secondary hazard to personnel / assets and provides sufficient time for personnel evacuation. Depressurization study is one of the mandatory studies in oil and gas for long time. Thus process simulator common used in oil & gas such as HYSYS, PRO-VISION, etc have built-in depressuring module which
specifically sued for depressurization study. HYSYS has been in the market for more than 15 years and it is well accepted by many users as it commonly known with its “real time” information and user friendly interface. One of section within the depressuring unit where user needs to provide information is the depressuring valve parameter page. The valve parameter page is basically to define the characteristic and vapor flow equation of depressuring device. Users are free to choose those seven (6) vapor flow equations (built-in), There are : See below image.
HYSYS default is [Fisher]. User may view the equations used for each of them by clicking the “Valve Equation Help…” button. See following image.
Out of 6 vapor flow equations, which equation shall be chosen and applied ? HYSYS recommends to used [Fisher] and [Relief] as these equations are more advanced than other valve equation. No doubt these equations are established and well accepted by many users. How shall these equations be applied in different kind of systems ? In my point of view, [Fisher] and [Masoneilan] equations will be used when controlled depressuring system. Common controlled depressuring application are depressuring of compressor and Mole-Sieve bed. It is very common the manufacturer of this equipment impose maximum pressure drop-rate (kPa/min) to protect their equipment. User may needs to selected a good control valves to limit the maximum pressure drop-rate whilst meeting maximum depressuring time limit of 15 minutes. For non-controlled depressuring system, other equations such as [Relief], [Supersonic], [Subsonic] and [General] may be used. [Relief] equation is recommended by HYSYS and can be used by setting the set pressure lower the FULL OPEN pressure (REMEMBER : Both setting shall be lower than the maximum expected pressure. Otherwise the valve will not “open”). I guess the main issue here is to provide the discharge coefficient (Kd) for orifice. Personally there may be some document out there which has recommended what type of Kd factor to be used. If any of user aware of it, please share in the comments column. [Supersonic] equation basically is a derivation from the [General] equation by considering Kterm and k equal as unity. This equation may be used in non-controlled depressuring system and it is for critical flow condition where backpressure is lower than system pressure. User may use this equation if the fluid characteristic is not well defined. User shall take note that there is a possibility where initial depressurization will be in critical flow condition and towards the end, backpressure may be exceeded the critical pressure. This will result some level of inaccuracy. This is very unlikely event and generally it is ignored. [Subsonic] equation is an equation which common used in non-controlled depressuring system and to handle subcritical flow system (backpressure higher than system pressure). This is the only equation built into HYSYS which can handle subcritical condition (as far as I aware but I guess [Fisher] and [Masoneilan] may be capable). HYSYS has not documented this. Thus it is not recommended to use. [General] equation is a well accepted equation which has been extracted from PERRY’s Chemical Engineering Handbook. The only parameters user required to input are the orifice Discharge Coefficient (Cd) and Orifice area (A). As in most of the case, a square-edged restriction orifice is applied for depressuring, (downstream of Blowdown valve, BDV), the discharge coefficient (Cd) is well defined in PERRY’s Chemical Engineering Handbook. See following image.
Generally the Reynolds number at the orifice throat is higher than 10,000, thus the Discharge coefficient is in the range of 0.6 to 0.65. Beware that HYSYS has recommended the used of 0.7 to 1.0. This may be true when the Reynolds number is in the range of 100 to 10,000. User shall always check at the end of study. Personally I would recommend to us 0.6 and reconfirm with Reynolds Number check by end of run. One shall remember this equation is applicable to Critical flow condition only. As in most cases, plant emergency depressuring will be critical flow, i am recommending to use this equation when you are dealing with noncontrolled depressuring. You comments and advices are welcome.
Depressuring Flow - Quick Manual Method Depressuring system is provided in Oil and gas, Gas & LNG plant, etc to evacuate the inventory from process system as fast as possible so that the reduced internal pressure stresses is kept below the rupture stress. This has been discussed in "Depressuring within 15 minutes no longer applicable ?". Nevertheless, quick depressuring may lead to other problem such as low temperature embrittlement, excessive noise and vibration, etc. Depressure a high pressure would lead to low temperature of depressured system and failure due to low temperature embrittlement. Higher the depressuring rate, lower the temperature can be experienced by depressured system. Thus, the restriction orifice (RO) downstream of Blowdown Valve (BDV)
in depressuring system primarily is to limit flow so that the temperature will not drop below the allowable lowest temperature limit of material. This has been discussed in "Don't misunderstood depressuring". Although depressuring shall be implemented within the shortest time possible, excessive depressuring may potentially lead to damage to equipment such as compressor seal, solid bed, etc. Thus, there are two type of depressuring as discussed in "Controlled and Non-controlled Type Depressuring". Nevertheless, it is emphasized again here, depressuring system shall be designed to bring plant to safe level without any tolerance. Many depressuring systems are designed to depressure the system within 15 minutes follow recommendation in API 521. Nevertheless, one shall take note that the 15 minutes is sort of arbitrary and may be good for some system and configuration. Thus, in most recent API STD 521, the requirement has slightly changed where a depressuring system shall be designed such that the stress induced by internal pressure of a system is lower than stress allowable by the system. This may lead to shorter depressuring time as discussed in "Depressuring within 15 minutes no longer applicable ?". Depressuring can be conducted using simple depressuring module in process simulator such HYSYS, PRO-II, etc. One shall understood there are limitation and accuracy issue in above mentioned depressuring modules and shall be used with care. There are other depressuring simulator such as LNGDYN by TECHNIP, BLOWDOWN by Imperial College, etc which are calculated rigorously and tested with real plant data. It is always advisable to use these simulator for specific case. Assumption In this post, a manual depressuring method is introduced. This method is first introduced by Grote and are derived base on following assumptions : i) Critical flow throughout entire depressuring process ii) Constant mass flow throughout entire depressuring process iii) System being depressured is maintained as gaseous throughout entire depressuring process iv) Constant temperature, molecular weight and compressibility Methodology Following is the derivation of the manual equation.
Concluding Remark Equation [5] may be used for manual depressuring if a system inventory (initial mass, M0), depressuring time (t), initial (P0) and final pressure (P) are are known. One shall check the assumptions are valid before it is used. This equation may be used as quick method to determine the depressuring flowrate for quick estimate, however it is not recommended during detailed design.
Additional Concerns in Controlled Depressuring Depressuring system is commonly provided in Oil and gas facilities to facilitate system inventory blowdown to a safe level within a reasonable time during emergency situation. Detail requirements may refer to API Std 521 section 5.20.1. Conventionally depressuring system consist of Blowdown Valve (BDV) with Restriction Orifice (RO) located some distance (about 600mm) downstream of BDV. Read "Why Restriction Orifice is some distance from Blowdown valve ?" to understand more the distance requirement. Beside, there are controlled and non-controlled Type Depressuring. Second type of non-controlled depressuring is a throttling device with control function into it. Recently some smart engineers have proposed to use the second type with throttling device to replace the conventional BDV (ball valve) plus RO type based on following reasons : * Ball valve opens too quickly which may cause bending of RO leading to size changes and higher flow on next use. * Ball valve does not throttle flow and opening will erode the ball & sealing. Globe valve, with soft seats, will have minimal damage and can also be used for pressure control. * Globe valve with control function will increase the blowdown rate with gradual opening in later stage Minimium (almost none) reported case in the failure of RO High flow leading to bending of RO and subsequently cause higher flow and low temperature issue of blowdown vessel might be one of the credible concern in conventional BDV plus RO system. Beside, erosion & cavitation occured at the RO possibily lead to larger RO. Althought this concern is valid, however there are minimium (almost none) reported case in the failure of RO in this regard. The could be credited to a infrequent use, proper blowdown rate determinination, correct material selection, properly calcuated RO bore size.
Excessive Flow When Control Function Failure Conventionally a valve with control function will be sized to open at 75-85%. With the characteristic of valve i.e. equal percentage and remaining 15-25% opening would results a flow of 2-3 time when it is FULLY OPEN. Employing a globe valve with control function, whenever the control function is failed and derive a maximum signal to open the glove valve fully, it will results excessive flow to the downstream disposal system. Large and costly disposal system is expected. Difficult Valve Characteristic Selection If a quick opening valve is selected to reduce the large flow during valve full open , this will partially cause inefficient control during low flow. Additional effort is required on valve characteristic selection. Reliability, Availability & Cost Effective Reliability and Availability of blowdown with control valve is another concern. Conventional BDV (ball valve) is spring to open, double solenoid valves, volume bottle, high SIL Blowdown system lead to increase safety integrity. Use of control valve may experience difficultites in meeting the safety integrity or high cost is incurred. Thus, a proper relaibility, availability and cost effective solution may have to considered. Concluding remark Above proposal certainly a good idea to be investigated in detail as there is a great potential in assisting system blowdown in shorter time and reduce overall safety risk. A proper discussion shall be included to address the issue as highlighted above in order to provide a cost effective but not tolerating any safety concerns.
Few Recommendation on Manual Blowdown Line
If you have worked on P&ID, you probably have seen an isolation valve with a globe valve across a pressure relief valve (PRV). See below image. What is this arrangement and what the purpose of installing them ? Many engineer may have aware of this purpose of this line. This arrangement is a manual blowdown line. It is normally used system inerting during start-up and system purging during shutdown maintenance. As the
source of the inventory is same as PRV i.e. vessel and the discharge location is same i.e. flare, thus it is generally drawn in P&ID as such. Similarly it also being arranged in parallel with PRV in actual installation. Some engineers may understand the main purpose of this arrangement but somehow mis-called it as PRV bypass. It is pretty same as the control valve (CV) bypass arrangement. In reality operator can operate the CV bypass and throttle the flow manually by the bypass globe valve while operator take out the control valve for maintenance purpose. However, operator is absolute not allowed to bypass the PRV. When designer write the operating manual, the correct terms shall be used to avoid any misunderstanding. This could be a minor error however sometime it is important. Some recommended features associate with this arrangement are as follow : * The isolation valve and globe valve shall be arranged such that no low pocket present as liquid possible present in low pocket promote corrosion * These arrangement install at high point so that liquid is sloping away from them * Maintain a distance i.e. 600mm between isolation valve and globe valve to avoid isolation valve stuck open during manual blowdown. See reasoning in "Why Restriction Orifice is some distance from Blowdown valve ?" * The recommended size of the line and valve is 2". If size smaller then 2" is used, concern of severe vibration of small bore connection (SBC) shall be addressed. * The short line downstream of globe valve to connection is recommended to design to MACH number lower than unity (1) i.e. 0.8 when the globe valve is fully open. This is to avoid operator inadvertently open the globe valve. * The downstream piping shall be sufficiently thick to ensure it is failed on acoustically induced vibration. * The isolation valve may be Normally Closed (NC). However, losing inventory due to inadvertently open of this blowdown valve is deem to be a concern, the isolation valve may be Car Sealed Close (CSC) or Locked Close (LC). * Prior to any blowdown, the operator may leave the upstream system settle out. This allow the internal fluid temperature cooled to minimum possible ambient temperature. Manual blowdown after settled out would lead to very low temperature downstream of globe valve. Thus the low temperature during manual blowdown shall be studied to ensure the material will not failed on low temperature embrittlement. * If the fluid is having high pour point, solidify or crystallize under minimum ambient temperature, etc, may consider insulating and/or heat trace the line. Concluding remarks Although above manual blowdown line is pretty simple, there are still features associate with it. Above is non-exhaustive list of features. If you find any special conditions, welcome your advice and comments. posted by Webworm, 3:26 AM | link | 0 Comments |
Why Restriction Orifice is some distance from Blowdown valve ? Very basic does not imply obvious...i really like this statement. Very often the answer is very fundamental and basic to us, somehow it always does not obvious to us.
Lets look at the above sketch. It is a typical arrangement for a Blowdown Valve (BDV) with Restriction Orifice (RO) , located downstream of the Blowdown valve. This arrangement typically used to depressurized a system inventory to a safe level within a limited time to minimized catastrophic scenario. Between the Blowdown valve and Restriction orifice, there 600 mm spool piece between them. What the main purpose of this 600mm spool piece ? Is this spool for straightening and smooth flow ? Is this spool to avoid secondary choke ? Guess what ? Reasoning...Major pressure drop will take place at Restriction Orifice, downstream of Blowdown valve during blowdown. Joule-Thompson (JT) effect results fluid temperature downstream of Restriction Orifice drops below subzero (Less than zero degree Celsius). Latent heat in the piping will transfer to cold fluid and slowly approaching this subzero temperature. The "coldness" will travel back to upstream of Restriction orifice and probably reaches Blowdown valve. It potentially cause the upstream Blowdown valve body temperature drops below subzero as well. Moisture from atmosphere will freeze at the Blowdown valve body and potentially cause the stem stuck at position. Operator may not possible to close the Blowdown valve after blowdown activities and may potentially lead to back flow. Those "general good engineering practice is to locate Restriction orifice, 600mm downstream of Blowdown valve.
