STUFFING BOX LEAKAGE CONTROL INTRODUCTION Compliance with environmental regulations requires the elimination of gas leakage from machines, especially when working with hazardous or toxic gases. In the field of reciprocating compressors, GE Oil & Gas has designed a special type of stuffing boxes, separation cover or intermediate stuffing box equipped with special rings for low pressures and inert gas buffers enabling to do 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 RECIPROCA RECIPROCATING TING COMPRESSORS In a reciprocating compressor, where the gas is processed in a cylinder, assuming that all mechanical seals (cylinder bottom and valve covers) are efficient, gas can only leak to the outside from the stuffing box which functions as a seal for piston rod. Figure 1 shows, the three potential leakage areas:
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• gasket (1) between the first first box, and the cylinder itself; • contact ssurface urface (2) between between adjacent adjacent rings rings housing boxes; • rings (3) and especially the last rings rings (4) downstream of the gas recovery, which work at low pressure. Other factors influencing leakage are the finish of the rod and of the contact surfaces between boxes, the piston rod run-out and the cylinder alignment. By applying a unique surface finish for both rods (reciprocating compressors modification kit 06) and boxes, appropriate plastic materials for seal rings (reciprocating compressors modification kit 03) and proper assembly of the different parts, it is possible to minimize gas leakages. However, even with a new stuffing box, leakage cannot be completely avoided the ring seal closure depends on gas pressure exerted on the exposed surface of the ring. Since conditions of insufficient pressure force can compromise the seal leaks can be eliminated only by adopting further measures.
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THE STRUCTURE WHICH GUARANTEES LEAK RESISTANCE The stuffing box To guarantee efficient sealing of the cylinder to the extension and frame, both when the seals are new and after normal wear, it is absolutely necessary to provide an exit path for the gas leaking from the rings. The most efficient approach is to inject (see Ffigure 1) inert gas into an appropriate location in the stuffing box (5), (if possible N2 or a gas which is environmentally benign at a pressure slightly higher than the gas recovery pressure. In this way, any process gas leakage is swept out and conveyed to a location for safe handling. In the extension there will be only an insignificant quantity of inert gas leaking from the buffer intermediate stuffing box and separation cover. With reference to figure 2, the extension can contain a small amount of inert gas or even process gas in the event of a stuffing box failure or annomolous back pressure from the gas recovery line. Therefore, it is necessary to equip the intermediate stuffing box (a) (in the case of machines with double compartment extensions) or the separation cover (b) (in the case of simple extensions) with an inert gas buffer system, as indicated above for the main stuffing box. In this case, the chamber adjacent to the cylinder is equipped with a vent to convey any gases to a safe area.
Low pressure rings In addition to the buffer system, a set of rings working as a positive seal for the buffer chamber is necessary to minimize the quantity of inert gas consumed. Indeed, contrary to intuition, it is more difficult to seal stuffing box components with rings working at low rather than high pressure. While the only problem at high pressure is wear resistance, for low pressures below 3.5-4 bar (recovery gas pressures) the problem is guaranteeing a perfect seal with the 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 rings could cause detachment of the rings from the box surface and hence gas leakage. Given that recovery pressures may be well under this limit, it is very important to use rings specifically designed for this application, that is rings with springs for an axial preload (1) to guarantee sealing at all conditions (Figure 3b). Therefore, Nuovo Pignone has designed a new low pressure ring system consisting of three components; two rings are tangential (2), the third consists of two parts (3) providing axial preload against the tangential rings by means of springs (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 gas buffer.
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STUFFING BOX LEAKAGE CONTROL SCOPE OF SUPPLY For each cylinder, the supply consists of the following items: • 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 separation 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 3 a) 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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Nuovo Pignone S.p.A. via F. Matteucci, 2 - 50127 Florence (Italy) Tel. +39-055-423-211- Fax +39-055-423-2800 E-mail:
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In this case, only three rings with springs housed in the last box are used (Figure 4). When a buffer gas is used, the sets of three rings are installed with opposing preload (Figure 5) so that rings create a chamber with high leak resistance for buffer gas injection. Types of buffers Usually, the maximum buffer pressure with standard materials is 6 bar g; the recommended pressure is 0.3 to 0.5 bar higher than the recovery gas pressure. Different buffer gas strategies are available to meet specific plant requirements and customer preferences: 1 - Constant pressure: buffer pressure is kept constant and higher than the maximum value of the recovery pressure. Recommended recovery pressure does not change very much. 2 - Variable pressure: buffer pressure maintained at a fixed differential with respect to the recovery pressure, therefore following variations in the recovery gas to maintain a constant over pressure. Recommended if recovery pressure undergoes large variations and to optimize nitrogen consumption. 3 - Variable pressure with respect to pressure in the recovery chamber: An electronic pressure transducer is connected to the recovery chamber of each stuffing box. This gives the actual pressure within each stuffing box recovery chamber including effects of losses in piping and ducts.
The pressures of each chamber are compared and the buffer pressure is set at 0.2 bar over the highest valve. This results in increased safety and reduced N2 consumption. 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. APPLICATION For older style stuffing boxes with metallic seal elements it is advisable to replace the stuffing box completely such as with the reciprocating compressor 03 kit. For new generation machines, it is only necessary to modify stuffing boxes downstream of the recovery chamber and ring housing boxes of the intermediate stuffing box or of the separation cover, to allow assembly of enclosed springs. On the contrary, for models HM, SHM and SHMB, because of dimensional limits, it is possible to assemble only one set of three rings with axial springs; therefore, the buffer chamber is manufactured with a tangential - tangential couple upstream of the recovery chamber and with a set of three rings towards the extension, as indicated in Figure 4.
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