Stuffing Box leakage reduction

gasket (1) between the first box, and the cylinder itself; contact surface (2) between adjacent rings housing

boxes; rings (3) and especially the last rings (4) downstream

of the gas recovery, which work at low pressure.

Other factors influencing leakage are the finish of the rodand of the contact surfaces between boxes, the piston rodrun-out and the cylinder alignment.By applying a unique surface finish for both rods (recip-rocating compressors modification kit 06) and boxes,appropriate plastic materials for seal rings (reciprocatingcompressors modification kit 03) and proper assembly ofthe different parts, it is possible to minimize gas leak-ages. However, even with a new stuffing box, leakagecannot be completely avoided the ring seal closuredepends on gas pressure exerted on the exposed sur-face of the ring.Since conditions of insufficient pressure force can com-promise the seal leaks can be eliminated only by adoptingfurther measures.

CM&UsCM&Us

GE Power SystemsOil & Gas

STUFFING BOX LEAKAGE CONTROL

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INTRODUCTIONCompliance with environmental regulations requires theelimination of gas leakage from machines, especially whenworking with hazardous or toxic gases.In the field of reciprocating compressors, GE Oil & Gashas designed a special type of stuffing boxes, separationcover or intermediate stuffing box equipped with specialrings for low pressures and inert gas buffers enabling todo the following: Eliminate gas leaks and convey hazardous gases to

safe handling areas Increase operational and personnel safety Comply with even the most restrictive environmental

laws

GAS LEAKS IN RECIPROCATING COMPRESSORSIn a reciprocating compressor, where the gas is processedin a cylinder, assuming that all mechanical seals (cylinderbottom and valve covers) are efficient, gas can only leak tothe outside from the stuffing box which functions as aseal for piston rod. Figure 1 shows, the three potentialleakage areas:

Low pressure ringsIn addition to the buffer system, a set of rings working asa positive seal for the buffer chamber is necessary tominimize the quantity of inert gas consumed.Indeed, contrary to intuition, it is more difficult to seal stuffingbox components with rings working at low rather than highpressure. While the only problem at high pressure is wearresistance, for low pressures below 3.5-4 bar (recovery gaspressures) the problem is guaranteeing a perfect seal withthe rod and especially with the housing box surface.Infact, in locations where the pressure between rings is low,during the intake stroke friction between the rod and ringscould cause detachment of the rings from the box surfaceand hence gas leakage. Given that recovery pressuresmay be well under this limit, it is very important to use ringsspecifically designed for this application, that is rings withsprings for an axial preload (1) to guarantee sealing at allconditions (Figure 3b).Therefore, Nuovo Pignone has designed a new low pres-sure ring system consisting of three components; two ringsare tangential (2), the third consists of two parts (3) providingaxial preload against the tangential rings by means ofsprings (1) housed on the inner surface of the box. Rings with axial preload are recommended for their high

efficiency at low pressure even when there is no inert gasbuffer.

THE STRUCTURE WHICH GUARANTEES LEAKRESISTANCEThe stuffing boxTo guarantee efficient sealing of the cylinder to the exten-sion and frame, both when the seals are new and after nor-mal wear, it is absolutely necessary to provide an exit pathfor the gas leaking from the rings.The most efficient approach is to inject (see Ffigure 1) inertgas into an appropriate location in the stuffing box (5), (ifpossible N2 or a gas which is environmentally benign at apressure slightly higher than the gas recovery pressure. Inthis way, any process gas leakage is swept out and con-veyed to a location for safe handling. In the extension therewill be only an insignificant quantity of inert gas leaking fromthe buffer intermediate stuffing box and separation cover.With reference to figure 2, the extension can contain a smallamount of inert gas or even process gas in the event of astuffing box failure or annomolous back pressure from thegas recovery line.Therefore, it is necessary to equip the intermediate stuffingbox (a) (in the case of machines with double compartmentextensions) or the separation cover (b) (in the case of simpleextensions) with an inert gas buffer system, as indicatedabove for the main stuffing box. In this case, the chamberadjacent to the cylinder is equipped with a vent to conveyany gases to a safe area.

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Figure 1

Friction force acting on the ringsduring piston rod movement

2 3 1 Gas leakage Gas pressure(cylinder side)

Figure 3b Figure 3a

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N2 N2

Figure 2

Nuovo Pignone S.p.A.via F. Matteucci, 2 - 50127 Florence (Italy) Tel. +39-055-423-211- Fax +39-055-423-2800E-mail: [email protected]/geoilandgas

STUFFING BOX LEAKAGE CONTROL

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SCOPE OF SUPPLYFor each cylinder, the supply consists of the followingitems: Stuffing box with set of three rings with axial preload. Intermediate stuffing box or separation cover with

set of three rings with axial preload. Modification drawings. Buffer instrumentation.Buffer instrumentation consists of:Stuffing box type 1, intermediate stuffing box and sepa-ration covers

a) inert gas filter;b) buffer pressure reducing valve;c) low pressure alarm pressure switch;d) safety valve for protection from buffer over-pressure.

Stuffing box types 2 and 3a) inert gas filter;b) variable set point buffer pressure reducing valve;c) pressure switch for low differential pressure alarm;d) safety valve for protection from over-pressure;e) maximum signal selector (only for case 3).

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In this case, only three rings with springs housed in the lastbox are used (Figure 4). When a buffer gas is used, the setsof three rings are installed with opposing preload (Figure 5)so that rings create a chamber with high leak resistance forbuffer gas injection.

Types of buffersUsually, the maximum buffer pressure with standard materialsis 6 bar g; the recommended pressure is 0.3 to 0.5 bar higherthan the recovery gas pressure.Different buffer gas strategies are available to meet specificplant requirements and customer preferences:1 - Constant pressure: buffer pressure is kept constant and higher than the maximumvalue of the recovery pressure. Recommended recovery pres-sure does not change very much.2 - Variable pressure: buffer pressure maintained at a fixed differential withrespect to the recovery pressure, therefore following varia-tions in the recovery gas to maintain a constant over pressure.Recommended if recovery pressure undergoes large varia-tions and to optimize nitrogen consumption.3 - Variable pressure with respect to pressure in the recovery chamber:An electronic pressure transducer is connected to therecovery chamber of each stuffing box. This gives the actu-al pressure within each stuffing box recovery chamberincluding effects of losses in piping and ducts.

The pressures of each chamber are compared and thebuffer pressure is set at 0.2 bar over the highest valve.This results in increased safety and reduced N2 con-sumption.

ADVANTAGES Elimination of gas leaks and hazardous environmental

conditions. High increase in operational and operator safety. High increase in seal efficiency at low pressure. Optimization of buffer gas consumption.

APPLICATIONFor older style stuffing boxes with metallic seal ele-ments it is advisable to replace the stuffing box com-pletely such as with the reciprocating compressor 03 kit.For new generation machines, it is only necessary tomodify stuffing boxes downstream of the recovery cham-ber and ring housing boxes of the intermediate stuffingbox or of the separation cover, to allow assembly ofenclosed springs.On the contrary, for models HM, SHM and SHMB,because of dimensional limits, it is possible to assem-ble only one set of three rings with axial springs; there-fore, the buffer chamber is manufactured with a tangen-tial - tangential couple upstream of the recovery cham-ber and with a set of three rings towards the extension,as indicated in Figure 4.

Figure 4