GEOil & Gas

Benefits

Stuffing Box Leakage Control

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Reciprocating Compressors

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Increased production Higher efficiency Compliance with environmental regulations Availability and Reliability Life extension

Customer benefits include:

Elimination of gas leaks into hazardous environments

Substantial increase in operational and operator safety

Significant increase in seal efficiency at low pressure environments

Optimization of buffer gas consumption

Increased seal element life due to superior control of the packing performance

What it is

Environmental regulations increasinglyrequire the strict control or completeelimination of gas leaks from plantmachinery. This is especially the casewhen working with hazardous or toxicgases. These regulations are beingsubjected to increased scrutiny bygovernment authorities responsiblefor inspections and enforcement, andmore restrictive legislation willcertainly appear in the future.GE Oil & Gas has designed specializedstuffing boxes, separation covers, andintermediate stuffing boxes equippedwith special rings suitable for lowpressure and inert gas buffers forreciprocating compressors thatprovide the following benefits: Conveyance of emitted gases to

safe areas, thereby eliminating gasleaks into environments wherethese gases could be hazardous

Considerable increase inoperational and personnel safety

Compliance with even the strictestenvironmental laws

If all the mechanical seals (cylinderbottom and valve covers) of thecylinders of a reciprocating compressorare operating correctly, the gas canonly leak to the outside from thestuffing box, which functions as a sealon the piston rod. There are threeprimary potential leakage channelsfrom the stuffing box (Figure 1):

Gasket (1) between the first box onthe cylinder side and the cylinderitself

Contact surface (2) betweenadjacent rings and housing boxes

Rings (3) and especially the lastrings (4) down-stream of the gasrecovery point, if any (i.e., thelowest pressure rings)

6

5 4 3

2 1

GAS + N2

N2

GAS

Cylinderside

Figure 1: Leakage control

How it works

The Stuffing BoxTo guarantee efficient sealing of thecylinder to the extension and frame,both when the seals are new andworn, it is absolutely necessary toprovide a stage pathway for the gas leaking from the rings.The most efficient approach is theinjection of an inert buffer gas suchas N2 (see Figure 1 item 5) into anappropriate compartment of thestuffing box, at a pressure slightlyhigher than necessary for gasrecovery. This forces the leaked

process gas and buffer gas mixtureto enter the recovery system (6) fromwhich it can be conveyed to a safearea. There will only be a minor quantity of inert buffer gas leakage from the extension.

Intermediate Stuffing Box &Separation CoverWith reference to Figure 2, theextension can contain a smallamount of inert gas or even processgas in the event of a stuffing box

failure or back pressure from the gas recovery line.Therefore, it is necessary to equip the intermediate stuffing box (a) - inthe case of machines with doublecompartment extensions - or theseparation cover (b) - in the case of simple extensions - with an inertgas buffer system, as indicatedabove for the main stuffing box.In this case, the chamber adjacent to the cylinder is equipped with avent to convey any gases to a safe area.

What it isHow it works

Figure 3b Figure 3a

Low Pressure RingsNotwithstanding the introduction of abuffer system, a set of rings thatprovide a seal for the buffer chamberis necessary to minimize the quantityof inert gas consumed.In fact, contrary to what might beexpected, it is more difficult to sealstuffing box components when therings work under low rather than highpressure conditions.While the only problem at highpressure is wear resistance, for verylow pressures the problem isguaranteeing a perfect seal not onlywith the rod, but especially with thehousing box surface. Sometimes thepressure between rings is low enoughto prevent proper functioning of therings. In this case, the friction force thatthe rod exerts on the rings, during theintake stroke of the frame end, couldcause their detachment from the boxsurface and result in gas leakage(Figure 3a).Given that the recovery pressure maybe well under this low pressure limit, itis important to use rings designedspecifically for this purpose. Such ringshave an axial preload applied throughsprings (1), which guarantee sealingunder all conditions (Figure 3b).GE Oil & Gas has designed new lowpressure rings consisting of threedifferent parts; two rings are tangential- tangential (2), while the third consistsof two parts (3) with an axial preloadfunction for tangential rings usingsprings (1) housed in the inner surfaceof the box.Axially preloaded rings are advisablefor their high efficiency at low pressureeven when there is no inert gaspressurization.In this case, only three rings withsprings housed in the last box are used.For cases in which a buffer gas is used,the sets of three rings are installed withopposing preload (see Figure 4) so thatthe rings create a chamber with highleak resistance for buffer gas injection.

Type of buffersUsually the maximum buffer pressure with standard materials is0.3 to 0.5 bar higher than the recoverypressure.Depending on the plant requirementsthe nitrogen buffer may be under:1) Constant pressure:

the buffer pressure is kept at aconstant level, higher than themaximum value of the recoverypressure. This is advisable if therecovery pressure does not changevery much.

2) Variable pressure with reference to pressure in the packing recovery chamber: the actual recovery

pressure is detected from the recovery chamber of each stuffing box through a suitable hole made in the box. In case of compressor with morethan one cylinder the pressure ofeach chamber is compared withsignal selectors, and the largestvalue is used to control the buffergas pressure. The buffer pressure is normallyregulated at 0.2 bar above themaximum stuffing box recoverychamber pressure. This system hasconsiderable advantages, includingimproved safety and reduced N2consumption.

a b

N2 N2

Figure 2

Friction force acting on the ringsduring piston rod movement

2 3 1 Gas leakage Gas pressure(cylinder side)

N2

Scope of SupplyFor each cylinder, the typical scope ofsupply consists of the following items: Stuffing box with sets of three rings

with axial preload Intermediate stuffing box or

separation cover with sets of threerings with axial preload

Any necessary modificationdrawings

Buffer Instrumentation:Buffer type 1, intermediate stuffingbox and separation coversa) inert gas filterb) buffer pressure reducing valvec) low pressure alarm pressure

switch

d) safety valve for protection from potential buffer overpressure

Buffer type 2 a) inert gas filterb) variable set point buffer pressure

reducing valvec) pressure switch for low

differential pressure alarmd) safety valve for protection from

overpressuree) maximum signal selector

ApplicationIn the case of old stuffing boxes withmetal seal elements, it is advisable toreplace the stuffing box completely.For modern machines, it is only

necessary to modify the stuffingboxes downstream of the recoverypoint and the ring housing boxes ofthe intermediate stuffing box, or ofthe separation cover to allowassembly of encased springs.For models HM, SHM and SHMB,because of dimension limitations it isonly possible to assemble one set ofthree rings with axial springs.Therefore, the buffer chamber ismanufactured with a tangential -tangential couple upstream of therecovery point, and with a set of threerings towards the extension.

g GE imagination at workGE Oil & Gasvia F. Matteucci, 250127 Florence - ItalyT +39 055 4272500F +39 055 4232800

Nuovo Pignone S.p.A.

E customer.service.center@ge.comwww.ge.com/oilandgas

COMK/MARK 839/II - Designed by: Studio Tre FasiPrinted by: Sagraf - 1-20082008 Nuovo Pignone S.p.A. all rights reserved

Figure 4

N2

N2

N2

N2GAS

+ N

2

VEN

T

How it works