天然氣增壓站經(jīng)濟優(yōu)化:管道與增壓站選型

Rainer Kurz Matt Lubomirsky

索拉透平公司， 加利福尼亞州 圣地亞哥 92123

1 Pipeline Sizing Considerations管線選型考慮

Kurz and Ohanian［1］evaluated different options for pipe diameters，pressure ratings and station spacing for a long distance pipeline.A 3 220 km(2 000 mile)onshore buried gas transmission pipeline for transporting natural gas with a gravity of 0.6 was assumed(Figure 1).

Kurz R 和Ohanian S［1］評估了對于長距離管線，管道直徑、壓力等級與站場間距的不同選擇。本文假定一條3 220 km(2 000 mile)的陸上埋地天然氣輸送管線，輸送氣體密度為0.6(圖1)。

Assuming that pipes will be available in2”diameter increments from pipe mills，the nearest even increments of the above-mentioned theoretical diameters were selected(24”，28”，and 34”for 152 bar，103 bar and 69 bar(1 bar=0.1 MPa)pressures respectively)and analyzed by varying the number of stations along the pipeline.The result of this refinement is shown in Figure 1，where present value is plotted against number of stations for each pressure level. The minima for each are shown in the chart with present value，total horsepower，and number of stations.In this study，the 69 bar pipeline has the lowest present value，thus would be the most cost effective solution.

Figure 1:Optimum number of stations and optimum maximum operating pressure(MAOP)for the 3 220 km(2 000mile)，560 000Nm3/h sample pipeline.The lowest cost configurations for each MAOP solution are marked(from［1］).圖1 一條長3 220 km(2 000 mile)、輸量為560 000 Nm3/h

假定來自制管廠的鋼管直徑以2”的間隔增加，選擇24”、28”與34”及其分別對應(yīng)壓力152、103 與69 bar(1 bar=0.1 MPa)的3 條管線，對沿線不同的布站數(shù)量進行分析。這樣細化研究的結(jié)果見圖1，圖1 中繪制了不同壓力等級下布站數(shù)量對應(yīng)的凈現(xiàn)值。每種情況的折合凈現(xiàn)值如圖1 所示，圖上反映了投資、總裝機功率、壓氣站數(shù)量。該案列說明，69 bar 管線的凈現(xiàn)值最低，因此是最經(jīng)濟的解決方案。

In actual practice，for commonality reasons，identical size units will be installed in the stations.In order to have identical power at each station，the station spacing will be adjusted(dependent on the geography)since the stations at the beginning of the line will consume more power than the stations at the end of the line due to the power required for fuel compression. Identical power at each location also depends on site elevation and design ambient temperature，which would define the site available power from a certain engine.

在實際應(yīng)用中，為了保持機組的統(tǒng)一，同一站內(nèi)應(yīng)安裝相同的機組。為了保持各站有相同的功率需求，需要根據(jù)地形調(diào)整站間距，由于沿途各站存在自耗氣，下游的輸氣總量小于首站及管道上游各站，因此管線首站比管線末站消耗更多的壓縮功。為保持相同功率而選擇的站址也受海拔與設(shè)計環(huán)境溫度影響，因為這些因素決定燃機的實際現(xiàn)場出力。

One of the key findings is，that the optimum is relatively flat in all cases.This meansin particular，that certain considerations may favor larger station spacing，with higher station pressure ratios，and higher MAOP in situations where pipelines are routed through largely unpopulated areas.

其中一個重要發(fā)現(xiàn)是，最優(yōu)方案對于其它方案來說其優(yōu)勢并不特別突出。這就意味著若管線穿過人煙稀少的地區(qū)則可以考慮增大站間距，提高站的壓比與MAOP(最大允許操作壓力)。

2 Typical Application典例應(yīng)用

For a case study we consider an international long distance pipeline. The total length of the line is about 7 000 km.The pipeline consists of two 42”parallel lines which turn into single a 48”line at the crossing of an international border. The pipeline design throughput is 30 billion Nm3per year and maximum operating pressure of this pipeline is 9.8 MPa.There are 10 compressor stations planned in one area，and over 20 stations in the receiving country.After first gas，it takes 5 years to build up to full capacity.

在案例研究中我們評估了一條跨國長輸管線，線路總長度為7 000 km。管線由2 根42”并行管線組成，在跨越國境邊界合并為1 根48”管線。管線設(shè)計輸送能力300 ×108m3/a，最大運行壓力9.8 MPa。在一個區(qū)域中設(shè)置10 座壓氣站，在接氣國家設(shè)置約20 多座站場。投產(chǎn)后，預(yù)計5 年達到管線設(shè)計輸送能力。

Figure 2:Impact of loss of one unit forthe 3 unit and the 2 unit scenarios圖2 對比3 臺壓縮機組與2 臺壓縮機組其中1 臺機組失效的影響

When we compare operations of the compressor station we need to recognize two main approaches.We can either operate with fewer of larger turbocompressor units(Case A，2 large units)or with a higher quantity of smaller turbocompressor units(Case B，3 small units). The following factors need to be considered when selection of either option is decided. In evaluating the system reliability and maximum throughput the impact of unit outages needs to be considered.If we were to consider two large 30 MW units the failure of one of them will result in 50reduction of power available whereas if we consider 3 smaller 20 MW units the failure of one of them will result in only 33power reduction.Figure 2 outlines the basic fact that，if the surviving units run at full load to make up as much flow as possible，the operating point for the Case B will be close to the highest efficiency island so the remaining on-line compressors will be working more effectively compared to the Case A when the single remaining large unit will be working in the stonewall area.It is obvious，that pipeline recovery time will be shorter in case B.

比較壓氣站運行方案時我們需要認識到兩種重要方法。既可以運行數(shù)量較少但功率較大的透平壓縮機組(案例A，2 臺大機組)，也可運行功率較小但數(shù)量較多的透平壓縮機組(案例B，3 臺小機組)。比選這兩種方案時，需考慮如下因素。在評估系統(tǒng)的可靠性與最大輸送能力時，需考慮機組失效所造成的影響。如果考慮2 臺30 MW 機組，其中1 臺機組失效，將導(dǎo)致可用功率減少50;然而如果考慮3 臺20 MW 機組，其中1 臺機組失效，將僅導(dǎo)致可用功率減少33。圖2 說明如果1 臺機組失效后，剩下的機組會盡可能滿負荷運行來保證輸氣量，案例B 中的壓縮機工作點將靠近高效區(qū)，而案例A中剩下的壓縮機將會運行在阻塞區(qū)附近。顯然，案例B的管線恢復(fù)時間更短。

Based on an analysis by Santos［2－3］，Case A can represent even more problems. The amount of gas that the single remaining 30 MW unit will have to process is so big that it will put this remaining unit into choke，and thus for practical purposes out of operation.The amount of fuel that the remaining unit is going to burn will not justify that negligible increase in head that this unit will provide. So，practically，when one larger turbocompressor will be out of operation，the second will have to be shutdown and the station will be bypassed.Station configurations with the single oversize driver and either no standby or standby on each second or third station are often advocated.The arguments in favor of this method are very high pipeline availability(99.5)and high efficiency(40～42)of the larger 30MW turbocompressor units.In fact，designing for a turbine oversized by 15will lead to normal operations at part load conditions almost all the time(99.5)where there will be negative impact on turbine efficiency and，as a result of it，increased fuel consumption. Another negative impact of this approach is that normally this pipeline would operate at lower than MAOP pressure，whereas the highest operational pipeline pressure produces less pressure losses and，therefore，lower requirements for the recompression power.The reason for that is the maintenance schedule for the turbocompressors on the stations with the single units without standby. In order to perform maintenance on these units the pipeline，linepack will have to be maximized up to MAOP，so that unit can be taken off line and the pipeline throughput will not be impacted.Therefore，the normal pipeline operations have to be based on a lower MAOP.Also worth mentioning is the pipeline capacity when considering the single turbocompressor approach.Many pipelines transport gas owned and produced by different commercial entities. As such the gas fields development time and gas availability depends on many technological and，lately，political factors that may potentially have negative impact on pipeline predicted capacity growth. In these conditions the single oversized turbocompressor will be either working into deep recycling mode until the expected amount of gas will become available or start operation with smaller capacity compressor stages which will subsequently require a costly change of the internal bundle.

根據(jù)Santos S P［2－3］的分析，案例A 可能帶來更多問題。由于剩下1 臺30 MW 機組需要處理的氣量太多導(dǎo)致機組出現(xiàn)擁塞，因此實際上不能正常運行。剩下機組消耗的燃料氣很多但卻只能提供微小能頭的增加。所以實際上，當(dāng)1 臺較大的透平壓縮機失效后，第2 臺機組將不得不停機，氣體只能越站輸送。通常情況下一般提倡壓氣站配置較大的驅(qū)動機(功率備用)且無備用機組，或配置合適功率的驅(qū)動機，在其后第2 或第3 個站場考慮備用機組。支持這種配置的理論是，管線有非常高的可用率(99.5)及30 MW 大機組較高的效率(40～42)。實際上，透平設(shè)計超出正常功率15的裕量就會使得燃機在幾乎所有(99.5)運行時間內(nèi)一直處在部分負荷狀態(tài)下，這不僅會影響透平的效率，還會增加燃料氣消耗。該方法的另一負面影響在于，這種配置還會使管線的運行壓力一直低于MAOP，我們知道管線的運行壓力越高，產(chǎn)生的壓損越小，再增壓需要的功率也越小。導(dǎo)致這個問題的原因是無備用的單機組的維護無法得到保證。為了對管線上的這些機組進行維修，需要使管道儲氣量在最高運行壓力下達到最高，然后才能停下機組，這樣管線的輸送能力才不會受到影響。因此，通常情況下管線都在低于MAOP 工況下運行。同樣值得一提的是單臺透平壓縮機組方案時管道的輸送能力。許多管線輸送的天然氣分別由不同單位生產(chǎn)以及擁有，這些氣田的開發(fā)時間進度與天然氣的外輸量由許多技術(shù)的、政治的因素決定，這些因素很可能對管線遠期達到設(shè)計輸量的能力產(chǎn)生負面影響。在這些情況下，單臺較大透平壓縮機要么會一直打循環(huán)運行直到達到預(yù)期輸氣量，要么開始時安裝一個處理低流量工況的壓縮機機芯，但是隨后換大流量機芯又會增加投資成本。

3 Fuel Comparison燃料比較

It is increasingly import ant to evaluate all seasons conditions when making a comparison between two different station layout cases.For the subject pipeline different design organization were involved in the pipeline feasibility study.One of them has used summer conditions only and came to the conclusion that larger turbines are preferred option. Another source used annual average conditions and came to the opposite conclusion. The reason for that was the fact that during winter，fall，and spring months，which cover total of 9 out of 12 months of operation，one of the smaller turbcompressors was put in the standby mode.Due to lower ambient temperature the amount of power available from the remaining 2 units was enough to cover the 100duties due to high compressor efficiency.This was not true for the Case 1(based on same explanation above)and both 30 MW units had to work in the deep part load with unsatisfactory turbine efficiency.The fact that operational mode became 2 +2 for Case B gave additional benefits worth mentioning. Since two turbocompressors were in standby mode there was an opportunity to do all maintenance work during this time of the year.It means that availability of this system becomes superior compare to Case A，especially if we were to consider summer months of operations.

比較兩種不同站場布置案例時，評估所有季節(jié)的工況越來越重要。之前所述的管道項目有不同的機構(gòu)參與了前期的管道可行性研究。其中一個機構(gòu)僅考慮了夏季工況，然后就得出結(jié)論認為大機組更優(yōu)。另一個組織則考慮了年平均工況從而得出了相反的結(jié)論。其原因在于，冬季、秋季與春季在1 年中共占了9 個月，這段時間內(nèi)其中較小機組中的1 臺則處于備用狀態(tài)。當(dāng)環(huán)境溫度較低時，且由于壓縮機的效率很高，剩下2 臺機組的出力就能覆蓋100負荷運行。以上論述對案例A 不適用(基于上面同樣的解釋)，2 臺30 MW 機組都在相當(dāng)?shù)偷牟糠重摵上逻\行，且燃機效率不佳。案例B 操作模式變?yōu)? +2 帶來的另一個好處也值得一提。由于2 臺機組處于備用模式，這就有機會在同一年對所有機組進行維修工作，這意味著系統(tǒng)的可用率遠遠高于案例A，特別是在考慮夏季工況的情況下。

4 Maintenance and Overhauls維護與大修

Another advantage of operating only 2 out of 4 units for a significant part of the year(i.e.9 out of 12 months)is the extended time between overhauls. Based on the calculations below，the total number of hours for each turbocompressor unit per year was reduced from 6 570 to 4 928 and，therefore，the time between overhauls could be extended.Based on 3 +1 units operate during 3 summer months and 2 +2 units operating during the rest of the year(9 months)，if the units were used so that they all ran exactly the same number of hours each year，each unit would run for 4 928 hours every year. Whereas if we account for 3 working units with one standby throughout the year the number of working hours will be as follows:8 760 × 3 units running=26 280 /4 units available=6 570 total hours per unit /year.

配置4 臺機組而大部分時間(如1 年中9 個月的時間)只運行其中2 臺機組的另一個優(yōu)勢是可以延長大修的間隔時間(日歷時間間隔而不是燃機運行時間間隔)?；谝韵掠嬎?，每臺燃壓機組每年的運行時間由6 570 h減少到4 928 h，因此，大修間隔將擴大。基于夏天3 個月3 +1 運行，其余9 個月2 +2 運行，如果機組每年運行的時間相同，每臺機組將運行4 928 h。然而如果我們整年3 +1 運行，運行時間如下:8 760 ×3 =26 280，每臺機組平均運行時間26 280 /4 =6 570。

Note that all units run for an equal number of hours to make the calculation simple. However，the customer could push lead machines to reach the agreed time between major inspections(TBI)first，so that all engines do not come up for overhaul at the same time，this would help with the overhaul cost，helping to distribute the overhaul cost over the 30 year cycle.We can even make step further and will see additional benefits of this approach. Accounting for the normal year around operation with 3 units on-line，each turbocompressor will get 6 570 ×30 year =197 100 required hours of operations.Where as considering 2 +2 setup for 9 months the total number of the required hours of operations reduced down to 147 840 hours.With modern turbines technology it is not uncommon to see that lifetime operation reaches 150 000 hours.It means that for the lifetime of this project(30 years)there will be no need to buy new set of equipment.This alone makes huge favorable impact on projects economics.

注意，我們假設(shè)每臺機組運行的時間都相同會使計算更簡便。然而，用戶需使第1 臺機組在重要檢測前到達規(guī)定運行時間(TBI)，因此所有燃氣輪機就不會同時到達大修，這有益于節(jié)約大修費用以及分配30 年的大修費用。我們甚至可以進一步分析，將會發(fā)現(xiàn)這種方式帶來的額外好處。3 臺機組運行，每臺透平壓縮機機組將運行6 570 ×30 a =197 100 h，我們假定9 個月2 +2，總的運行時間降至147 840 h。對于現(xiàn)代透平技術(shù)，工作壽命到達150 000 h 并不是不常見。這意味著對于這個項目的全壽命運行期間(30 年)不需要再購買新的設(shè)備，僅這一點就對項目的經(jīng)濟性產(chǎn)生了巨大影響。

5 Station versus System Availability站與系統(tǒng)可用率

It is important to recognize the difference between station and pipeline availability.For economic assessments，misunderstanding this issue can lead to the wrong conclusion.Station availability calculations are easy，straight forward and based on simple statistical equations.It is easy to see that fewer units on a compressor station will yield higher availability，assuming the threshold for availability is 100of the flow.But is this true for the entire pipeline system?The answer is not easy and requires additional investigation including extensive hydrodynamic analysis using of the statistical methodology. The Monte Carlo method(Santos，2009)has proved to be the good methodology to determine the pipeline system availability.The statistical portion consists of generating multiple random cases of equipment failure on single or two consecutive compressor stations. The hydrodynamic portion will calculate the maximum throughput that pipeline is available to carry when these failures occur. Based on this extensive and in-depth analysis it can be shown that availability of a pipeline，configured with smaller multiple units，delivers better overall results. The main reason for that outcome is the fact that shutdown of the smaller unit makes lesser impact on the behavior of the entire pipeline.Of course，to have fair results，the availability of the single turbocompressor unit，either smaller 22 MW or larger size 30 MW was identical.It is easy to understand that in our particular case the availability of the station setup with smaller units(case B)was greatly enhanced because of the presence of extra standby unit during winter and fall /spring months when stations setup has 2 +2 configuration.

認識到站場可用率與管線可用率之間的不同是十分重要的。對于經(jīng)濟評估而言，若錯誤理解這個問題將得出錯誤的結(jié)論。站場可用率計算基于簡單的計算公式，簡單直接。顯而易見，壓氣站機組較少則可用率將更高。但對于整個管線系統(tǒng)是這樣嗎?答案沒有這么簡單，還需要更多的調(diào)查，包括通過使用統(tǒng)計學(xué)的方法對管道進行大量的水力分析。Monte Carlo 法［3］已被證實是一種較好的用來確定管線系統(tǒng)可用率的數(shù)學(xué)方法，統(tǒng)計部分包括先假設(shè)多種隨機的在單個或兩個連續(xù)的壓氣站發(fā)生設(shè)備故障的狀況;水力計算部分則會計算出管線在這些機組失效情況下的最大輸氣能力?；谶@個擴展與深入分析可知，管線配置多臺小機組時整體可用率會更好。得出這個結(jié)論的主要原因是小機組停機對整個系統(tǒng)產(chǎn)生的影響較小。當(dāng)然，為了有公正的結(jié)果，我們必須假設(shè)單個透平壓縮機組的可用率，不論是22 MW 或30 MW 機組，是相同的。這樣就很容易理解使用案例B 的配置站場的可用率更高，因為在其余9 個月2 +2 運行時，壓氣站有額外的配用機組。

6 Effect of Large Unit Shutdown大機組停機的影響

Examples of the vulnerability are demonstrated based on a typical pipeline scenario with 4 stations.Each station has 2 compressor trains without spares. If one unit in station 2 is lost，the pipeline flow is reduced by 12.However，the same 12flow reduction can be maintained by also shutting down the surviving unit in station 2.This is due to the necessarily inefficient operation of the surviving unit in station 2，which is forced to operate in choke.If both units are shutdown，stations 3 and 4 will be able to recover the flow，but at a much higher overall efficiency. Thus，shutting both units down reduces the pipeline fuel consumption compared to the scenario with only one unit shut down in station 2. The point of this example is，the failure of one of two large units in a compressor station has more significant consequences than the failure of a smaller unit in a station with three or more operating units.Or，in other words，scenarios with 3 ore more units per station without spare units tend to have a higher flow if one of the units fails，or has to be shut down for maintenance，than scenarios with 2 units per station without spare units.

以1 條設(shè)有4 座壓氣站場的典型管線為例，每個站場有2 臺壓縮機組且都不安裝備用機組。如果第二個站場的1 臺機組停機，整條管線的輸送能力將降低12;然而，關(guān)閉第二個站場剩下的1 臺機組，整條管線的輸送能力也降低12。這是因為第二個站場剩下機組的低效率運行，這種低效率運行將使機組運行至阻塞工況。如果2 臺機組都停機，第三個站場與第四個站場將能夠恢復(fù)流量，這樣整個管道就會運行在更高的效率從而降低總體燃料消耗量。因此，同時關(guān)閉2 臺機組會比只關(guān)掉第二站的1 臺機組在總體上節(jié)約燃料氣消耗。這個案例也說明，一個壓氣站2 臺大機組中1 臺機組失效比一個帶多臺小機組的站場中1 臺機組失效的影響更大。或者換句話說，每個站場有3 臺或更多機組，在1 臺機組失效或停機維修時，不配置備用機組，可以保持更多流量。

7 Conclusion結(jié)論

The paper has illustrated the different influence factors for the economic success of a gas compression operation.Important criteria include first cost，operating cost(especially fuel cost)，capacity，availability，life cycle cost，and emissions.Decisions about the layout of compressorstations such as the number of units，standby requirements，type of driver，and type of compressors have an impact on cost，fuel consumption，operational flexibility，emissions，as well as availability of the station.

本文闡述了對一個壓氣站經(jīng)濟成功運行的不同影響因素。重要指標(biāo)包括一次性投資、運行成本(特別是燃料氣消耗)、能力、可用率、全生命周期的成本與污染物的排放。壓氣站的整體設(shè)計，如機組數(shù)量、備用機組、驅(qū)動方式，壓縮機類型等對成本、燃料氣消耗、機組運行的靈活性、排放以及站場的可用率均有影響。

Reference

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