020102_150

02.01.02 - 150

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Process engineering

Pumps, turbines and compressors

Compressors (4)

The design and operation of a multi-stage piston compressor will be discussed inthis lesson. Reference will also be made to the pressure-volume diagram (p-Vdiagram). The Thomassen C45-4 compressor has been selected to help explainthe design of a compressor. The operation, drive, lubrication oil and coolingsystem of this compressor will be discussed.In a separate chapter, the discharge and suction valves used in this type ofcompressor will be described. The lesson concludes with a brief description ofthe procedures to start up and shut down a compressor installation of this type.

Contents of the lesson

1 Piston compressor

2 Operation of a piston compressor

3 Thomassen C45-4 compressor

4 Construction of the compressor

5 Valves used in piston compressors

6 Construction of the electric motor

7 Lubricating oil system

8 Cooling system

9 Start-up and shut-down of a compressor

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Process engineering / 02.01.02 - 150

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Lesson

1. Piston compressorPiston compressors are be divided into two main groups:- single-stage compressors;- multi-stage compressors.

As the name implies, a single-stage compressor requires one step, thecompression stroke, to bring the gas to pressure. In a multi-stage compressor thistakes place in two or more stages.

Another distinction can be made concerning the operation of the pistoncompressors to be discussed in this lesson, the:- single-acting compressor;- double-acting compressor.

With a single-acting compressor, only one side of the piston is used to compressthe gas.With a double-acting compressor, gas is compressed on both sides of the piston.

Another classification can be made according to the design:- vertical compressor;- horizontal compressor.

The most striking features here are the positions of the cylinders with respect toone another, namely vertical andor horizontal. Vertical compressors have theadvantage that a small base plate is sufficient, and that there will be relativelylittle wear on the pistons, piston rings and cylinders.With this design, however, there is the risk of liquid leaking into the cylinderwhich could lead to serious damage.Horizontal compressors do not run this risk because the suction valves arealways mounted on the lower side of the cylinder.However, the cylinder, piston and piston rings may experience greater wear as aresult of the weight of the horizontally moving piston.

In the petrochemicals industry, horizontal piston compressors are preferred.

The operation of a horizontal, double-acting, multi-stage compressor will bediscussed in the sections below.

- single-stage compressor- multi-stage compressor

- single-acting compressor- double-acting compressor

- vertical

- horizontal

- greater wear

- preference


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2. Operation of a piston compressorThe operation of a compressor is illustrated schematically in figure 1. At thestart of the discharge stroke (A), the suction valve closes. The air or gas drawnin cannot escape yet and is initially compressed. This compressing continuesuntil the pressure is somewhat higher than the pressure downstream of thedischarge valve. This is generated by the compressed gas that is already present(in the discharge vessel) plus a spring which is always present. The result is thatthe discharge valve opens (point B). The (slight) pressure increase can be clearlyseen in the pressure-volume diagram, as shown in figure 1.

During the now following discharge stroke (BC) the compressor supplies gas orair to the system in which a constant back pressure prevails.

The discharge valve closes at the end of the discharge stroke, reached at point Cand as shown in figure 1. However, there is a 'clearance pocket' in the cylinder.This clearance pocket is located between the piston and the cylinder cover. Thispocket is filled with compressed gas. This quantity of gas or air must firstexpand, in other words, drop to a pressure that is somewhat lower than thepressure in the suction line, before the suction valve at D will open and fresh gascan be drawn in. It is obvious that as the clearance pocket becomes larger at thesame pressure, point D moves further to the right. After expansion, this volumewill be proportionally larger. The peaks at B and D in the diagram are caused bythe fact that a certain differential pressure is required to open the valve than isrequired for 'simply' opening and then keeping the valve open.

Question 1What is meant by the term clearance pocket?

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Figure 1Pressure-volume diagram (p-V diagram) of the gas in the cylinder

- discharge stroke- compression

- clearance pocket

- initial expansion

- peak


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Question 2What does the surface area enclosed by A B C D represent?

It can be seen from this that just as with the discharge stroke (BC in the diagram)when gas is only emitted across a part, the suction stroke is also only partiallyeffective, namely across DA, whereas the total stroke proceeds from E to A.Point D is dependent on different factors, including:- the size of the clearance pocket (this is the pocket between the piston and the

cylinder cover, when the piston is in its extreme left position);- the type of gas;- the extent of cooling of the compressor;- the piston velocity;- the magnitude of the discharge pressure.

3. Thomassen C45-4 compressorWithin this series of lesson we will discuss a frequently used 4-cylinder, double-acting piston compressor of the Thomassen C45-4 compressor type.The compressor is supplied with H2-rich gas at a pressure of ± 10 bar from aplant in which this gas is a by-product (see figure 2).Three of the four cylinders (cylinders 1, 3 and 4) compress this gas to a 30-barhydrogen system, after which it is conducted to an absorber to recover any lighthydrocarbons that might still be present. The fourth cylinder (cylinder 2) draws aquantity of hydrogen gas out of this 30-bar system via a gas-lquid separator andboosts this to a 50-bar distribution system.The pressure in the systems is controlled by means of spill-back valves. Toensure that this quantity is as small as possible, cylinders can be either lined upor take off-line. Air coolers lower the gas temperature and thus the volume ofthe compressed gas.

Question 3After the first stage, the gas is conducted to an absorber. Indicate why this onlytakes place after this first stage.

- double-acting piston compressor

- H2 = hydrogen gas

- boost

- return


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5539-150-002-P

Figure 2Hydrogen system

4. Construction of the compressorThe compressor is a boxer-type compressor. In a boxer-type compressor thecylinders are located in a horizontal plane and mounted opposite one another onboth sides of the crankshaft (see figures 3 and 4). Figure 3 gives a top view of atwo-stage compressor. One of this compressor's cylinders is shown in detail infigure 4.

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Figure 3Top view of a 4-cylinder, two-stage compressor, as shown in figure 2 and in detail infigure 4

The compressor is driven by an electric motor. Each cylinder is double-acting.Therefore, the left side as well as the right side of the compressor piston are usedto compress the gas (or the air).

- boxer type


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The 50-bar cylinder is of a sturdier design. It is intended to be able to absorbmore efficiently the forces associated with the higher pressure. The forged steelcrankshaft is supported at both ends as well as in the centre between the twopairs of cylinders (by means of bearings).The connection between the crankshaft and the pistons is formed by a pistoncross-head system. A drive rod connects the cross-head to the crankshaft and apiston rod forms the connection to the piston and the cross-head. In this respect,the compressor is therefore very similar to an internal combustion engine. Astuffing box around the piston rod prevents any gas leaking into the carter.The stuffing boxes are designed as a pair with a space between them which has ableed system connected to a 'safe location' and the atmosphere. This space iskept at a slight overpressure with a nitrogen purge.The cylinders are surrounded by cooling water jackets that are connected to anclosed cooling water circulation system. All the discharge valves in the 30-barcylinders are designed in pairs. The suction valves can be lifted by levers(lifters) so that the capacity of the compressor can be controlled by opening andclosing a pair of valves in one of the cylinders. The discharge and suction valvesare designed as plate valves. These will be discussed in more detail in thislesson.The valves in the 50-bar cylinder are single valves. The suction valves are alsoprovided with lifters. Table 1 contains the technical data for the compressor.

Table 1Technical data for a Thomassen C45-4 compressorspeed Number of stagesRequired powerGas

Total number of cylindersPrinciple of operationBore hole (mm)Stroke (mm)Number of suction valves per cylinderNumber of discharge valves per cylinderGas suction temperature °CGas discharge temperature °CSuction volume (m3/min)

370 rpm2 (3 * LP, 1 * HP)3084 kW (nominal)H2 rich

Low pressure High pressure3 1double-acting double-acting450 350330 3302/2 1/12/2 1/145 35120 8294.454 19.987

Question 4Give a reason why the manufacturer may have chosen a boxer configuration forthe cylinders.

- cross-head system

- stuffing box

- nitrogen purge- cooling water chambers

- lifters

- plate valves


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Figure 4Front view of a Thomassen C45-4 compressor


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5. Valves used in piston compressorsThe inflow and outflow of the gas in the cylinder take place via the suction anddischarge valves. These valves are designed as free floating valves that are keptin equilibrium by the pressure of the gas and the spring force. The valves areusually designed as plate valves and contain a steel plate, but more frequentlythey are made from alloyed steel. The thickness of these valves varies between 1and 2 mm. The advantage of this type of valve is the small mass and thereforethey have a lower chance of slamming to the seat. With gas valves the danger ishigher than with liquid valves because there is no liquid to provide a brakingaction.

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Figure 5Cross-section of a suction valve

A suction valve for a compressor is shown in figure 5.

The design is such that a discharge valve is produced if the bolt, the nut and thering are switched over to be in the opposite direction.

This construction can be seen in figures 6 and 7 of the valves of a Thomassencompressor.

- freely floating valves- plate valves

- small mass

- braking action

- switching over


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Figure 6Detail of a suction valve for a Thomassencompressor

Figure 7Detail of a discharge valve for aThomassen compressor

6. Construction of the electric motorThe electric motor that drives this compressor is a 6000-volt, three-phase motoroperating at a speed of 370 rpm. The motor drives the compressor directly, i.e.there is no interconnected gearbox. The electric motor is cooled by a closedcooling air circuit, with this air in turn being kept at the right temperature bymeans of a water cooler.The compressor / electric motor combination is equiped with a turning device,driven by a small electric motor, which is separated from the compressor /electric motor in normal operation by means of a claw coupling. The turningdevice is provided to turn the compressor to remove any liquid present before itcan be started and the lubrication system commisioned. As long as the turningdevice is connected, it is impossible to start the compressor because thecompressor issafeguarded with a switch in the electrical start circuit. A handcrank makes it possible to turn the claw coupling into the right positionforcoupling to the compressor. The bearings of the electric motor are lubricated bytwo small lubrication oil pumps fitted in the bearing seat. These are driven bythe main shaft via a worm wheel transmission. The technical data are given intable 2.

Table 2Technical data forthe electric motorTypePowerVoltageCurrent

three-phase induction motor3860 kW6000 V, 50 Hz192 A (unloaded)480 A (loaded)

- no gearbox

- turning device

- claw coupling


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7. Lubricating oil system

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Figure 8Lube oil system

The compressor is equipped with a lube oil system which consists of anelectrically driven oil pump, an oil cooler and an oil filter designed with a sparer.The compressor frame functions as an oil sump. The oil pump draws from thecompressor frame via a strainer.Using an electric heating element the oil can bebrought up to temperature before starting. The discharge pressure of the oilpump is kept to a maximum of 5 bar by a relief valve. The oil passes via the oilcooler and the filters to a reducing valve that keeps the pressure to a maximumof 2 bar for lubrication of the compressor. The lube oil system is protected withalarm and trip functions. The cylinders and stuffing boxes are lubricatedseparately by a multi-point oil injection system. The pump for this system isdriven by a small electric motor. Each lubrication point has its own smallplunger with a capacity control and a sight glass.

- heating element

- multi-point oil injection system


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8. Cooling systemThe cooling system (see figure 8) for the compressor comprises two coolers anda thermostat valve. Depending on the measured temperature at (c), a larger orsmaller amount flows through the cooler (b) and a smaller or larger amount (a)bypasses the cooler. The thermostat valve ensures that the lubricatingoil remainsat the correct temperature by passing a larger or smaller amount through thecoolers.

9. Start-up and shut-down of a compressorStart-upAfter the various liquid levels and temperatures have been checked andcorrected if necessary, the cooling water pump and the lubricating oil pump canbe started. The flow of cooling water through the various cylinder jackets ischecked by means of sight glasses.The multi-point oil plunger pump must be started approximately 5 minutesbefore starting up the compressor in order to lubricate the pistons and thestuffing boxes.All the suction valves must be lifted to allow the compressor to operateunloaded.The compressor must now be turned a few rotations to free the cylinders fromany liquid. This liquid can be drained off in the pulsation dampers. The rest ofthe pipe system and the liquid separators also have to be drained from anyliquid. The turning device is detached by means of the claw coupling and thenthe main electric motor is started. After the air coolers (see figure 2) have beenswitched on, the compressor can be loaded. This takes place by activating thesuction valves in the correct sequence. This sequence is important to ensure thatthe compressor is loaded uniformly.Follwing the start-up the lube oil and cooling water temperatures and levels haveto be checked. Also the current drawn by the electric motor is an importantparameter and must not be too high.

Shut-downThe compressor must be unloade in the reverse order by means of valve lifters,after which the main motor and the multi-point lubrication oil injection isstopped.After a certain amount of time, the lube oil and the cooling water pumps can bestopped.

Question 5Why is a turning device fitted?

- thermostat valve

- free of liquid

- claw coupling

- temperatures- level


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SummaryThis lesson provided an introduction into the subject area of piston compressors.The operation of a horizontal, double-acting, multi-stage compressor wasdiscussed in detail and the lubricating oil system and the cooling system weredescribed. The lesson was concluded with a discussion of the procedure forstarting up and shutting down a compressor.


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TestExercises1. Give a description of the Thomassen C45-4 compressor type.

2. Why is the return flow to be kept as small as possible?

3. Into which 3 groups can compressors be categorised in practice?

4. How can piston compressors be divided even further, for example?

5. In what situations is multi-stage compression used?

Answers to the questions in the lesson1. The clearance pocket means the space between the piston and the cylinder

cover at the instant when the piston is in its highest position.

2. The surface area is formed by the product of 'pressure' and 'volume'. The

unit is determined from the multiplication [ ]32 m*

mN

��

��

� = [Nm] = [Joule]

This unit is the unit of work.

3. At a pressure of 10 bar, the light hydrocarbons present are still in the gasphase. After the first compression stage, these are now largely condensed togive a liquid and the liquid can be easily washed out.

4. With a boxer configuration the compressor is well balanced. In addition, thelength of the crankshaft is minimal, and thus the length of the compressor.

5. Using a turning device, the compressor can be turned after a standstill to freethe cylinders from liquid and the cylinders and stuffing boxes can be 'pre-lubricated' using the multi-point oil injection system.


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Answers to the exercises1. The Thomassen C45-4 compressor is a 4-cylinder, double-acting piston

compressor of the boxer type.

2. Compressing a gas and then returning it to the suction pressure again is notenergy efficient. This cannot be completely avoided if the pressure is to becontrolled well. By lining up or taking off-line, this return flow can be keptas small as possible, so that energy can be saved.

3. The compressors used in practice can be categorised into:- piston compressors;- centrifugal compressors;- rotating positive displacement compressors.

4. A distinction can be made between single- and double-acting compressors.In addition, compressors can also be designed as:- single-stage compressors;- two-stage compressors;- multi-stage compressors.

5. Multi-stage compression is used if higher pressures have to be attained. Insuch cases, the compression can no longer be reached in a single stage. Thefinal compression temperature would then increase to an unacceptable level.The lubricants used could then form an explosive mixture with thecompressed air.In addition, as a result of thermal expansion 'fretting' and seizing ofcompressor components irreparable damage could be caused to thecompressor.Moreover, it can be demonstrated scientifically that using intermediatecooling, which is possible with multi-stage compression, will lead to savingsin terms of the compression work required.

Problems and assignmentsAnswer and send in for correction

1. How is the Thomassen C45-4 compressor lubricated and cooled?

2. In your own words, describe the way in which a compressor of this type hasto be started up.

3. Explain why a vertical compressor experiences less wear compared to ahorizontal compressor. Where does this wear occur in particular inhorizontal compressors?

4. Which type of compressor runs the greatest risk of damage caused by thepresence of liquid? Explain your answer.


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5. On which factors does the ratio between the effective suction stroke(DA, figure 1) and the theoretical suction stroke (EA, figure 1) depend?

6. Why is the distance BC called only 'partially effective' in the pV diagram infigure 1?

7. Why is the 50-bar cylinder in the installation as shown in figure 2 designedto have a sturdier structure?

8. Explain why (ring) valves slamming with 'gas pumps' more frequently thanwith liquid pumps.

9. Draw, with reference to figures 5, 6 and 7 in this lesson, a suction valve anda discharge valve of a piston compressor.

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