Dry Gas Seal Manual

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GE Energy Oil & Gas Customer Training

DRY GAS SEAL SYSTEM

TO WHOM IT MAY CONCERN This manual is intended for training purpose only and must not be used to operate the equipments. For this purpose please refer to JOB O&M compressor manual. Manual content is restricted to OMIFCO personnel only and must not be disclosed to any other people.

INTRODUCTION Slide 1

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GE Oil & Gas – Global Services

All the mechanical systems that realize a transformation of mechanical energy into thermodynamics energy, and vice versa, consist of moving and fixed components, both when we consider driver machines, like turbines and driven machine, like centrifugal compressors. Moving and fixed parts in a machine imply the need to realize some clearances to avoid contacts between them. The presence of these clearances between closer regions at different pressure determines a gas leakage, that we define Internal Leakage. When it occurs inside the machine and External Leakage. When it occurs from the inside to the outside of the machine. Therefore it is necessary to realize some systems that reduce as much as possible these leakages. Normally Labyrinth Seals are used for internal leakages and Gas Seals for external leakages. In Slide 1 we can recognize rotating and stationary components of a typical Centrifugal Compressor: Stationary Components: Casing ( A ), Diaphragm Bundle ( B ), Journal Bearing ( H ), Thrust Bearing ( I ), Seals ( M ). Rotating Components: Shaft ( C ), Impellers ( D ), Balancing Drum ( E ), Thrust Collar ( F )

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Slide 2

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The dry gas seal is a system made by a cartridge which is fit at both sides of a centrifugal compressor. It prevent the process gas from going out of the machine. This loss occurs because of the clearances existing between the shaft, rotating part, and the casing, stationary part.

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Slide 3

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During the last years the gas seals have replaced the oil seals in the sealing system of a centrifugal compressor because of three important differences:

1. System simplicity: The gas seal system is simpler than the oil seal one. In fact we do not have pumps, tank, over-tank and degassing tank of oil and all the instrumentation that we find in the oil system.

2. No process gas contamination: The gas seal works with the same compressor process gas, so we do

not utilize oil that can contaminate the gas

3. Increased efficiency: The sealing is done by creating a gas film instead of an oil film, in order to reduce the losses of power due to friction.

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Slide 4

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All Dry Gas Seals are made by two main seal rings; the rotating one and the stationary one. The sationary ring has a flat and smooth inner surface which is located just opposite to the rotating one. On the rotating ring surface instead we find some grooves having a particular shape. When the compressor is not running and depressurised the two rings touch each other because the springs behind the stationary ring push it against the rotating one. When the compressor is pressurised the gas fills in the chamber behind the stationary ring producing a force which pushes it towards the rotating one. This system guarantees the sealing also when the compressor is not in operation, see Slide 5

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Slide 5

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So when the rings are in contact we have some small chambers created between the stationary ring flat surface and the grooves on rotating ring. When the shaft runs the gas is pushed inside the grooves, see Slide 6, and it is subject to a small compression. When the shaft reaches a speed of some hundred rpm the gas pressure inside the grooves is able to win the force produced by the springs and displaces the stationary ring in order to create a little gap between the rings. In this way the rings are not in contact anymore, so we do not loose any power for friction due to the contact but we find a gas leakage. The leakage anyway is very little because of the small entity of the gap.

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Slide 6

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During the operation of the machine the gas enters in the grooves and produces the lift force.

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Slide 7

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Clearance during operationClearance during operation ~ 5 micron~ 5 micron

Grooves depth ~~ 10 micron

Human hair : 50 ÷75 micron

MAIN CHARACTERISTICS

Normally the size of the gap varies from three to five micron, while the grooves depth is about 10 micron. You can compare this values to the average diameter of a human hair to understand the operating condition of this system.

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Slide 8

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GROOVES SHAPE

The groove design is a logarithmic spiral. In Slide 8, computer animation shows that there is an optimum groove angle that will generate the maximum lift during operation. By contrast a radial slot will generate lift but this may be insufficient to ensure that the sealing gap is kept constant all time. This may increase the risk of friction particularly during transient conditions. It can also be seen that pressure generation reduces when the groove angle is more acute.

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Slide 9

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Slide 9 shows how small are the grooves on the rotating ring.

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Slide 10

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UNIDIRECTIONAL GROOVES

BIDIRECTIONALGROOVES

We can have two different types of grooves: unidirectional and bi-directional grooves. The unidirectional ones can operate only in one rotating direction, while the bi-directional one can work in both directions. The bi-directional grooves are used only if there is a real risk of inverse rotation of the compressor, during transient operations. OPERATING FEATURES As we have seen the gas seal is not able to completely avoid the gas leakages but due to such a small clearance between the rings they are strongly reduced. The lost gas flow leakage depends from the operating condition parameters; some of them are shown in Slide 11.

11

Slide 11

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OPERATING CONDITIONS PARAMETERS DEPENDENCE

- Gas type

- Pressure

- Temperature

- Speed

- Clearance between rings

- Materials

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Slide 12

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Slides 12, 13 and 14 show some examples of gas leakage vs. some operating parameters. Slide 12 shows how the leakages increase when speed and rings size increase.

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Slide 13:

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Slide 13 shows to see how the leakage varies according to the nature of the gas. The leakage increases by increasing the speed and reducing the temperature.

14

Slide 14

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In Slide 14 you can see how the lost of power changes due to the friction during the seal operation. It is evident that the lost power increases when speed, seal dimension and operating pressure increase. OPERATING STABILITY The clearance between the stationary and the rotating rings, on different operating conditions, is calculated in order to meet two different needs: - Minimize the gas leakage - Minimize the temperature between the rings due to the gas friction The needs are opposite to each other: we have to realize a small gap to reduce the leakage and a big gap to reduce the gas friction, and so the temperature, on the rings. The solution is to reach a compromise in order to have a low leakage and the lowest possible temperature. Therefore it is indispensable that during the operation the gap is kept constant to guarantee the design gas leakage and low rings temperature. To meet this objective the system is self regulated. In fact it is able to restore the design gap values whether they are modified by both external and internal stresses. During machine operation we can have a gap variation due to: - Variation of the dynamic operating conditions in the axial direction - Variation of the dynamic operating conditions in the transversal direction

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Slide 15

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Fource

Gap

Fp

Fs

hD h1h2

We have just seen that on operating conditions two forces are applied on the stationary ring. They keep it in self equilibrium: - Fp, force due to the pressure gradient generated by the grooves on the rotating ring - Fs, total springs force. Slide 15 shows the behaviour of the forces vs the rings gap size. It is clear that the operating point in design conditions is the intersection between the curves, point hD, where the forces are equal.

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VARIATION OF DYNAMIC OPERATING CONDITIONS IN AXIAL DIRECTION Look at what happens when the shaft is subject to an axial displacement due to vibrations. If the gap increases, point h1 Slide 15, the force due to the pressure gradient is smaller than the springs force, thus the stationary ring will be moved towards the rotating one in order to reduce the gap up to the design value. On the other side if the gap reduces, point h2 Slide 15, the force due to the pressure gradient will result bigger than the springs force and will move the stationary ring far from the rotating one. This will increase the gap up to the design value. Slide 16

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CLEARANCES VARIATION DUE TO

AXIAL SHAFT DISPLACEMENT

Slide 16 shows what we have described on the top of the page.

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VARIATION OF DYNAMIC OPERETING CONDITIONS IN TRANSVERSAL DIRECTION The stationary ring during operation can be subject to a rotation in vertical plane due to the action of pressure and temperature. Due to the pressure action on the upper part, the stationary ring should be subject to a rotation on a vertical plane with an increase of the gap, as you can see on Slide 17. The system comes back at design conditions in the same way we have seen for the axial displacement: by the increase of local gap, the force due to the pressure (Fp) gradient is smaller than the springs (Fs) force, so the last one creates a torque that rotates the stationary ring towards the rotating one in order to reduce the gap up to the design value. Slide 17

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DISTORTION DUE TO THE PRESSURE ON THE UPPER SIDE OF STATIONARY RING

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Slide 18

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EXAMPLE OF TEMPERATURE FIELD ON PRIMARY SEAL

P = 40 bar 13.000 RPM

In the same way we will have a stationary ring rotation due to the temperatures field distribution between the rings, as you can see on Slide 18.

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Slide 19

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THERMAL DISTORTION DUE TO THE TEMPERATURE INCREASING ON THE STATIONARY RING INTERNAL SIDE

These irregular temperatures field distribution determine a ring deformation with an increase of the local gap. The system comes back on design condition in the same way we have seen for the axial displacement: with the local gap increase, the force Fp is smaller than the Fs, so the last one creates a torque that rotates the stationary ring toward the rotating one in order to reduce the gap up to the design value, see Slide 19.

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MATERIALS The previous considerations highlight the influence of pressure and temperature on the seal operating conditions, underlining the big importance of the materials. The materials should be chosen in order to reduce the distortion. In particular they should have:

- High modulus of elasticity, to reduce the pressure deformation - High thermal conductivity, to remove the heat and reduce the thermal distortion

In the next three slides you will see some typical materials utilized in the dry gas seal. Slide 20

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MATERIALS UTILIZED FOR THE RINGS

seatSeal face

Secondary seal

spring

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Slide 21

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MATERIALS UTILIZED FOR THE SPRINGS AND ACCESSORIES

COMPONENTS

22

Slide 22

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MATERIALS UTILIZED FORSECONDARY SEALS

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GAS SEALS TYPES The sealing of the gas in a centrifugal compressor is usually done by a cartridge containing a couple of “gas seals” called tandem set. The first couple is placed in the inner side of the compressor and it is called primary seal while the one in the outer side of the compressor is called secondary seal, see Slide 23. Slide 23

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SECONDARY SEAL

PRIMARY SEAL

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Slide 24

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When the compressor works with a gas mixture (including some noxious components) you may find “triple” seal, as shown on Slide 24.

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Slide 25

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SECONDARY SEALTERTIARY SEAL

PRIMARY SEAL

The two seals, primary and secondary, are assembled in one cartridge fit on both sides of the machine right before the journal bearing. Behind the gas seal cartridge, on the external side, you may find the tertiary seal, (Slide 25) The aim of the tertiary seal is to avoid that the lube oil that comes from the journal bearing gets to the gas seal. If the oil reaches the seal it will seriously damage it. The tertiary seal can either be a labyrinth made of aluminium alloy or carbon rings. Later on we will see the operating of this seal.

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Slide 26

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Rotating part of a gas seal with rotating ring, Slide 26

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Slide 27

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Stationary part of a gas seal with the stationary ring and the springs, Slide 27

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Slide 28

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Labyrinth tertiary seal, Slide 28

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Slide 29

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Hardened sleeve on Shaft

Carbon

Springs

Tertiary seal made of carbon rings, Slide 29

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Slide 30

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Slide 30 shows the position of the gas seal and the tertiary seal on a centrifugal compressor. This arrangement is the same on both sides of the machine.

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Slide 31

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Compressor SuctionCompressor

DischargeSuction shaft end

Discharge shaft end

Balancing Line

Balancing Piston

For proper dry gas seal operation it is necessary to have an auxiliary plant that feeds the gas seal with the seal gas. Before starting with the dry gas seal explanation we have to understand how the compressor is oriented in the above drawings. When we look at Slide 31 we understand the shaft orientation is univocally determined by the presence of balancing drum (Red Square). An important role is also played by the balancing line (green Square): this one allows both compressor sides to reach the same pressure. Therefore the seal pressure value is the same in both sides and it is equal to the suction pressure value and so we find the suction pressure behind the balancing drum. Both dry gas seal work under the same condition: with suction pressure on one side of the seal and atmospheric pressure on the other side.

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Slide 32

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Feed gas Circuit

Nitrogen Circuit

Vent Circuit

The dry gas seal plant is composed by three different subsystems:

• Seal gas feed circuit (Red square Slide 32) • Vent circuit (green square Slide 32) • Buffer gas circuit (light blue square Slide 32)

Later on we will see in detail each subsystem.

33

Slide 33

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Dry Gas Seal System - Primary Seal

Seal Buffer Gas System Input

To feed the gas seal we extract gas either from the discharge pipe or from an intermediate compressor stage or an other extraction directly in the discharge pipe. Our purpose is to stop the suction pressure so in every intermediate compressor stage there is a pressure value higher than the suction one. The extracted gas is sent to high efficiency filters which guarantee the gas clearness. After the filtration process the gas is sent to the seal system by means of the “ Seal Buffer Gas System Input”, see Slide 33.

34

Slide 34

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A AB

K

In this way the A chambers are completely filled in with the seal gas which has a higher pressure than the gas inside K chambers. Now the seal gas which is inside the compressor will have on one side the dry gas seal and on the opposite side the B’s shaft end labyrinth seals. The B’s shaft end labyrinth seals are designed to have a radial clearance in order to guarantee that about 95% of the total gas injected in the A’s chambers goes back into the compressor, see Slide 34.

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Slide 35

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A AB

CC

K

The dry gas seals are designed in order to avoid the ring friction. In fact during the normal working the seals rings (stationary and rotating one) are detached from each other by the gas lift. Thanks to this detachment a certain amount of gas flow is able to cross the seal and arrives in the C anular chamber. The total amount of seal gas able to go through primary seal ring is about 5 –7 %, see Slide 35. The C chambers are very important because they are directly connected to the primary vent by means of which the gas is sent out of the compressor in an controlled atmosphere or directly to the flair. The vent line is instrumented with valves and gages. In order to allow the operators to adjust the vent pressure and monitor the system. The vent instruments also send electric signals to the main controller to shut down the unit if the working parameters assume critics values. In proper working conditions the primary vent pressure should be between 0.3 – 0.7 BarG. Later on we will go through all the necessary actions to adjust that value. The presence of under pressure gas C chambers guarantees the proper gas feeding to the secondary seal ring. Under certain specific circumstances where the gas could be dangerous or when we are working with high pressure process gas we have to use dry gas seal whit a nitrogen buffer gas injection. The nitrogen is sent between the primary seal ring and the secondary one, see Slide 36

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Slide 36

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A

C

D

E

When we use nitrogen buffering gas inside the dry gas seal we will also have a labyrinth seal “ D “ which act as a barriege between the E chamber and C chamber. The Nitrogen inject pressure is higher than the C chamber gas pressure. The D labyrinth seal radial clearance is designed so that half of the total injected nitrogen will be able to reach the C chamber. In the C chamber we will have a mixture of nitrogen gas and seal gas. This mixture directly goes in the primary vent. In the E chamber there will only be nitrogen. Passing through the secondary seal the nitrogen reaches the secondary vent it is discharged in atmosphere.

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Slide 37

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F

The secondary vent is directly connected to the annular chamber F, see Slide 37. There also arrives the nitrogen that comes from the tertiary seal.

38

Slide 38

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Hardened sleeve on Shaft

N2 Supply

Bearing sideSeal

side

0,5 Bar

To 2nd vent

If the gas leakageincrease, the differentialpressure drops down under 0,5 Bar

The tertiary seal purpose is to stop the oil that comes from the bearing. The nitrogen is sent into the tertiary seal at a certain pressure regulated by PCV. Later on we will see in detail this system. When we look at Slide 38 we easily understand how the tertiary seal works. The gas that goes out from the tertiary seal on the bearing side meets the lube oil that comes from the bearing. The gas reaches the atmosphere through the bearing vent.

39

Slide 39

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Lube oil from Bearing

N2 Inlet

Tertiary seal

We can have two different types of tertiary seal:

• One is made of carbon rings directly in contact with the compressor shaft (new version, see Slide 38)

• One uses the labyrinth seals (old version, see Slide 39). In the last one the nitrogen is injected in a central channel directly machined on the seal.

40

Slide 40

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N2 Buffer Gas

N2 Tertiary Vent

Primary Vent

Bearing Lube Oil Drain

CEF

Proces Gas

Seal Gas inlet pipe

Bearing Vent

Secondary Vent

A

Slide 40 shows how the dry gas seal cartridge appears when it is installed in its place.

41

Slide 41

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1

2

3

In this specific case the seal gas is extracted directly from the compressor discharge pipe and it is sent to the high efficiency filter, see Slide 41. In the green square we have the gas pressure value (100.7 BarG), the temperature value (134°C) and the gas flow value (44 Kg/h). The filtering battery consist of two different filters, one in operation and the other one in stand by. When the differential pressure transmitter PDT 4746 reads high differential pressure across the filter battery, we can manually switch the gas path whitout having to shut down the unit. The three way valves (red square) are the ones able to do this job. When the filters are clean PDT 4746 will read a differential pressure very close to zero. As the filter becomes dirtier and dirtier the differential pressure will increase. The seal gas differential pressure alarm value is 1.5 BarG. At this point we have to send the gas to the stand-by filter and replace the dirty one. In parallel to the transmitter we also have the PDI 4758 that is a local differential pressure indicator arranged in the dry gas seal panel. To switch the filters the following procedure must be done:

• Verify that the PDI 4758 reads the same differential pressure value read in the control room. • If the vales are different from each other verify the measure chain. • If the values are the same open the disc valve 1 and 2. • Open valve 3 in order to allow the gas to fill in the stand-by filter. • Switch the valve position (red Square). • Close valve 3 and replace the dirty filter.

42

Slide 42

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1

After the filters, see Slide 42, we find PCV 4744 which regulates the gas pressure that is sent to the seals. This valve operates a trim pressure regulation. In parallel we find an orifice, whose purpose is to guarantee a constant seal gas flow for the dry gas seal under any working condition. The PCV control is made by a measure chain that measures the differential pressure across the labyrinth seal number 1. This value is read by the transmitter PDT 4744. The signal goes to the control logic inside the control panel. This regulates the PCV opening degree in order to keep the differential pressure across the labyrinth 1 constant. This value has to be around 3 BarG. This differential pressure value guarantees that in the A annular chamber (see Slide 35) there is enough gas to make the gas seal work properly. In the measure chain we can also find PDI 4757; it measures the same value read by PDT 4744. PDI 4757 is an indicator and it is located in the dry gas seal main panel arranged close to the unit. In the same panel we have PI 4745 which reads the pressure value in the K annular chamber(see Slide 34 and 35). PI 4745 is a very important instrument and we will see it again during the trouble shooting phase.

43

Slide 43

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AN2 Buffer gas Lines

BBN2 Tertiary Seal Lines

The gas does not go in the compressor through the labyrinth seal reaches the C annular chamber directly connected to the primary vent, (see Slide 35). The primary vent is essential for the operators. In fact by watching the gas pressure and its flow through the vent they understand if the is working properly. A safety valve PSV 4701 set at 12 BarG is installed in the vent line. The valve allows the line depressurization in case of sudden seal break down. The transmitter PSXH 4732 is the high pressure shut down switch regulated at 5 BarG. When the shut down for high vent pressure occurs the control logic depressurizes the compressor. The local gauge PT 4733 is installed in the main seal panel on the base-plate and it is very useful to check the vent pressure. The needle valve FV 4735 is used to manually adjust the vent line pressure. In order to have the seal work properly it is necessary to close or open that valve up to reach a differential pressure across it between 0.35 a 0.75 BarG read by PDI 4734. To help the operator adjust the valve opening degree we can check FIT 4733, that reads the gas flow throughout the vent. FIT 4733 is set to raise an alarm for high gas flow when it becomes higher than H = 5.55 Kg/h. FIT 4733 will also raise an alarm for low flow when it becomes lower than L = 1.16 Kg/h. The secondary vent is not instrumented. Inside the vent we find only nitrogen gas and some oil vapors which are not dangerous and can be released directly in the atmosphere.

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Slide 44

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A

B

The N2 line feeds nitrogen to the tertiary seal. The tertiary line stops the lube oil that comes from the journal bearing and tends to go into the dry gas seal. The filters battery removes all the gas impurities to avoid seal damaging. PDT 4761 is a differential pressure transmitter installed up stream and down stream of the filters to check the level of filter clearness. The transmitter raise an alarm when the differential pressure approaches 1.5 BarG. In parallel to the transmitter we have a differential pressure gauge, PDI 4721, which is located on the main seal panel in the base-plate. The filters battery consists of two identical filters, one in operation and the other one in stand-by. To switch the filters we use the same procedure described for the dry gas seal filters battery. After the filters we find the PCV 4722 that is set to maintain a down stream pressure of 3BarG. The diameter of orifice FO 4728, installed in parallel to the valve, is designed to feed the correct gas flow to the line so that the valve can make a more precise trim regulation. Then the line splits into two different nitrogen buffer lines, A and B, see Slide 44. The A line sends nitrogen at 3 BarG to the E annular chambers. The B line reduce the N2 pressure up to 1 BarG by means of the orifice FO 4727 and the valve PCV 4724 and send it to the tertiary seal (see Slide 43).

45

Slide 45

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Trouble Shooting

In this section we will go through the most frequent and typical problems that could occur to the dry gas seal and we will examine all possible solutions.

46

Slide 46

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About 80% of the dry gas seal failure is due to contaminated seal gas. Slide 46 shows a series of data to highlight this aspect.

47

Slide 47

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Another typical failure occurs when the shaft rotate in the opposite direction to the normal one or when the seal cartridge is installed in the wrong side. We can also have seal gas failures when a reverse pressurization occurs, as shown in the Slide 47.

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Slide 48

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The presence of water, either liquid or steamy, inside the seal gas result into two different kinds of seal failure. In one case we will have rust in the rotating disc and this will produce a change in the grooves profile. In the other case we will observe ice formation caused by the temperature reduction. This is due to gas expansion during the seal crossing. Ice formation will produce an increasing of seal gas consumption because the ice produces rings misalignment. Moreover the ice could also scratch the carbon ring surface producing serious seal failure.

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Slide 49

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If Buffer Gas Is Wet theExpansion Can Produce Ice

Slide 49 shows the rings misalignment due to ice formation.

50

Slide 50

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Both an inaccurate cartridge installing and a superficial machine operation can produce seal damages. For example if during a shut down, the depressurization occurs too fast, the dry seal gas internal O-Rings can be damaged, see Slides 51, 52, and 53.

51

Slide 51

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In this picture the compressor is not pressurized yet and so the O-Rings are not under pressure effect.

52

Slide 52

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The compressor is now pressurized, all the dry gas seal O-Ring are under process gas pressure.

53

Slide 53

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During the normal running the seal gas fills the little holes on the external surface of the O-Rings. If the machine depressurization speed is too high the gas will go out from the holes to quickly and will break the O-Ring profile.

54

Slide 54

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Clearance between

carbon ring and spring

beat

The little O-Ring pieces will go between the stationary ring and its support as shown in the red circle Slide 54. These pieces will not allow the axial relative movement and this will cause the seal to remain open. This will result in an increase of seal gas consumption.

55

Slide 55

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The oil contamination is another very important cause of seal failure.

56

Slide 56

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Also the tungsten ring can be damaged but it is very rare because the carbon ring is softer than the tungsten one.

57

Slide 57

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The stationary ring cam be easily damaged by the gas impurities. This phenomena can occur when the filters are dirty.

58

Dry Gas Seal P&I Trouble shooting By the dry gas seal circuit monitoring and analysis it is possible to understand not only whether the dry is working properly but also if the compressor is working properly. All the following speeches are referred to the Slides 42 and 43. High filter differential pressure To check the filters state, the operator must read and record the differential pressure transmitter value. The alarm value is set at 1.5 BarG. If in the control room the PDT 4746 raises an alarm for high differential pressure, first of all an operator must go to the field and check whether the pressure indicator PDI 4758 is reading the same differential pressure value. If both values are equal then the stand by filter must be put in service. The same procedure must be followed for the nitrogen battery filters. High/Low differential pressure on the primary vent When the differential pressure read by PDI 4734 is out of the normal working range (0.35 – 0.75 BarG) the operator will need to act on the needle valve FV 4735: if the pressure is lower than the 0.35 BarG the valve FV 4735 must be closed while if the pressure is higher than 0.75 BarG the valve must be opened. Always remind that the dry gas seals must have a certain back pressure (0.35 – 0.75 BarG) to work properly. If PDI 4734 gives back a differential pressure value lower than the normal and closing the needle valve FV 4735 anything happens a performances calculation must be done. When the performance analysis is good but we still have low primary vent pressure, we have to consider that this may be caused by an opening in secondary seal ring which allows a large amount of gas to leave the compressor from the secondary vent. The pressure in the primary vent will drop down dramatically. The secondary vents are not instrumented and so it is not possible to measure directly what happens inside. If we have an increase of the PDV 4744 opening degree without any pressure variation in the PI 4745, followed by an increase in the primary vent and in the primary vent differential pressure, recorded by PT 4733 e PDI 4734, we understand that the seal is not working properly. When the vent pressure reaches 5 BarG the machine governor shuts down the compressor and opens the station vent to depressurize the unit. Increasing the thrust bearing temperature When the thrust bearing active side temperature increases it can be due to a balancing drum labyrinth seal failure. Sometimes this phenomena occurs after high level vibrations caused by a compressor surging. Under these conditions the labyrinth cannot guarantee the gas stopping and so the pressure behind the balancing drum will increase reducing the load it can absorb. As a consequence we will have a higher load in the thrust bearing and so a higher temperature. Normally the control logic shuts the machine down for high bearing temperature when this value is around 128° – 129° C and in any case lower than 130° C. Given the above seal circuit conditions we might experience an increase of PDV 4744 opening degree. In fact the control logic will tend to open the valve in order to maintain constant the differential pressure up stream and down stream the shaft end labyrinth seal, measured by the PDT 4744 transmitter (green labyrinth in Slide 58). The pressure value read by PI 4745, normally very close to the suction one, will tend to increase. Also the primary vent pressure might increase. If the shaft end labyrinth seal has been damaged (red circle Slide 58) there will be an increase of internal recycles and the compressor efficiency will drop down dramatically. Also the valve PDV 4744 opening degree will be higher than normal because the transmitter PDT 4744 will force the system to open that valve: as a result of pressure decrease across the labyrinth. In this case the thrust bearing temperature will not increase (see Slide 58).

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Slide 58

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Dry Gas Seal Manual