EXTREME CONDITION PROCESSING
RAYMOND W. HITESHUE U. S. Bureau of Mines, Pittsburgh 73, Pa.
T
HE U. S. Bureau of Mines has used high pressure bench scale
and pilot plant equipment for a number of years in connection with coal hydrogenation. This report describes some salient experience that may assist those engaged in similar high pressure work. .f
,LP"tel"
In high pressure operations
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Compressor packing performance depends on concentricity, lubrication, type of packing, and finish of rod and stuffing box lead-tin metallic packing, when properly installed and lubricated, prevents seepage of low molecular weight gases
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Valve leakage in multistage gas compressors can be minimized by filtering the gas to prevent solids from entering the compression chambers
t Figure 1.
\Pocking
[Pbt Sn)
Compressor packing arrangement in high pressure pilot plant
Pump packing problems are greatly simplified by use of a "floating" type rod and adequate lubrication Packing leakage during pumping of slurries can be eliminated by injecting a continuous stream of clean liquid into the mid-point of the stuffing box (this technique minimizes deposition of solids at packing interface) The hazard of overpressuring because of plugged protective devices can be overcome b y making the protective unit an integral part of the pump body
Gas compressors and pumps are perhaps the most troublesome mechanical components of a continuous high pressure unit, especially with respect to packing and valve performance. Possible types of packing for multistage compressors include ring, spacer ring, and rod designs. Usuallv. the first or lower two From holder Blank stages are provided with a ring design similar Inert gas to that employed in i n t e r n a l c o m b u s t i o n engines. The experimenter will rarely experience difficulty with this type packing. However, leaks do occur with the spacer-ring dePressure sign, which is usually found in the third or To unit fourth stage. As the gas is normally vented to the first stage, abnormal leakage in the spacer-ring section will be indicated by an increase in discharge pressure on the first stage. Such leakage is usually due to excessive clearance between "spacers" and their companion rods; c l o s e - f i t t i n g (drive fit) spacers will rectify this condition. Leakage in the higher or rod-packed stages can be prevented by maintaining finish, concentricity, and lubrication. The surface finish of both the stuffing box and the rod should be 60 microinch or better, and the two components ahould be I M I designed to obtain absolute concentricity of the assembly; misalignment between these Blowdown trap elements distorts the packing. Adherence to these principles will result in satisfactory stuffing box performance, provided reasonably designed packing is used. The Bureau has Figure 2. Installation of multistage gas compressor in high pressure had success with Vee packing, fabricated from pilot plant I
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May 1956
INDUSTRIAL AND ENGINEERING CHEMISTRY
838
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
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Figure 3.
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2
High pressure pump head
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u Scale, inch
Ball type Figure 4.
Poppet type
Check valves for high pressure pump unit
chrome-tanned leather or neoprene-asbestos and with conical and cylindrical packing fabricated from soft metals, such as tin-lead type-alloys. A recent design, involving the use of metallic packing for hydrogen service at 10,000 pounds per square inch, is shown in Figure 1. Gas leakage is eliminated by maintaining a continuous flow of oil from the mid-point of the packing section to the compression chamber. Clearances between the cylindrical packing elements and the rod are minimized by deforming the packing before operation. Leakage in compressor valves is usually caused by marring or pitting of the sealing surfaces. Corrosive agents, such as hydrogen sulfide, frequently cause pitting, but most damage may be attributed to abrasion by entrained solids. Particles of oxidized iron may originate by spalling from the suction piping or from the lower-stage castings. An appropriate filter on the inlet streams and adherence to a regular valve inspection program will minimize faulty valve performance.
To flare or
Blowdown v a l v e s
S h u t - o f f valves
P Figure 5. 836
High pressure pump installation
Multistage compressors must be considered in the light of possible hazards caused by compressing flammable or toxic gases. Figure 2 illustrates a typical compressor installation of the Bureau of Mines. Each compression stage is provided with a gage, a blowdown valve, and a safety valve. Blowdown valves are frequently opened during operation to wash away any solids that might have been deposited in the compressor valve pockets. Gases from safety or blowdown valves should be discharged into a separate vessel, and the effluent gases should be filtered before admission to the compressor. The objective of this arrangement is to prevent possible recontamination of the unit by solids and spent lubricant. As regards safety, a vent packing box should bo installed on the atmospheric Bide of the main drive rod to prevent leakage of combustibles into the operating area. Another desirable safety technique is to uae an interlocking pressure switch to prevent operation in the event of zero or low suction pressure. This feature, of course, guards against inadvertently compressing air into a gaseous system. hlultistage compressors may be purged by running on inert gas for a period corresponding to 20 or 30 volume changes during which time all blowdown valves, except the last, should be opened. The important design variables contributing to successful pump operation are, in general, similar to those required for dependable compressor operation. Figure 3, depicting a pump for 10,000 pounds per square inch gage, illustrates these requirements. The illustrated unit can handle from 50 cc. to 50 pounds per hour of liquid or dense (45y0solids) slurries, depending on the size of the (interchangeable) plunger or rod. The latter is equipped with a "collar button" end which permits lateral movement to maintain concentricity of rod, stuffing box, and packing in the event of misalignment of the axes of the pump head and the crosshead. The surface finishes of the stuffing box and rod are 60 microinch or better. Vee-type packing is employed and is provided with a lantern ring to permit insertion of a suitable grease lubricant. Figure 4 shows a poppet and a ball valve that may be used with t h i s particular pumpthe poppetusually with clean liquids and the ball with concentrated s l u r r i e s or high viscosity liquids. I n t h e poppet d e s i g n , a phosp h o r o u s bronze v a l v e h a s been used with an 18-8 s t a i n l e s s (Type 303) seat with a differential a n g l e of 2 to 3 degrees. The ball d e s i g n use8 a hardened ball (low chrome) i n conjunction with a line-type seat, f a b r i c a t e d Threaded sleeva from stainless Type 410 or 416 Figure 6. Spring loaded high and hardened to pressure connection about 350 Brinnel.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 48, No. 5
EXTREME CONDITION PROCESSING
,Dish arge
-Discharge
Valve plate
Valve plate
Sect
Seat
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\High
\High
pressure
Figure 7.
pressure
High pressure safety valves
A typical high pressure pump installation is shown in Figure 5. Use of a safety valve between the pump and the first shutoff valve in a plant is considered mandatory in all installations. A safety valve on the low pressure or suction side of the pump is also considered good practice when stop valves are required between the inlet of the pump and its reservoir. This safety valve protects low pressure piping in case of leakage through the check valves of the pump. Before starting reciprocating pumps, the two blowdown valves (Figure 5 ) should be opened and the two shutoff valves leading to the high pressure system should be closed. With this arrangement, the pump can be primed safely since the effluent is routed to an atmospheric zone. After priming, the pump should be stopped and the blowdown valves closed. The shutoff valves may now be opened and the pump started. I n view of the present interest in pumping slurries, a brief resume is given of our experience with coal-oil slurries. To avoid settling in suction lines, feed material must be circulated at high velocities across the suction part of the pump. Pumping rates must also be high enough to provide sufficient velocities within the pump head to prevent settling. When velocities within the pump cannot be increased above the settling velocity, settling may be avoidable by using smaller particles or by increasing the viscosity of the slurry. Stuffing box leakage may become severe in slurry work owing to abrasion by solids of the sealing faces of packing. This trouble can be eliminated by maintaining a small continuous flow of lubricating oil or any suitable clean liquid into the lantern ring of the packing assembly. Slurries may also block or plug lines leading to pressure protective devices, such as
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Pumps and compressors
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Control room
\Reinforced concrete1 I foot thick
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Figure 9. May 1956
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Floor plan of high pressure pilot plants
safety valves or blowout disks. This hazard may be eliminated by locating the safety valve within the pump head, as noted in Figure 3. Leakage at high temperature joints or a t joints subjected to cyclic flows of high temperature fluids is minimized by using a reasonable operating schedule together with a proper arrangement of piping or tubing. For example, heating should take place with constant gas flow, and pressurizing and depressurizing should take place slowly. Helically shaped lines between hot joints are also effective in minimizing expansion stresses. The connector shown in Figure 6 is useful at points subjected to cyclic stresses; it permits the tubing seat to follow dimensional changes that may occur at the seal point because of thermal shock. At points subjected to extreme temperatures and pressures, welded socket joints have out been quite useful. SatisA f a c t o r y techniques have now been developed for use of l/d- to C/&nch 0.d. stainless tubing. Leakage of safety valves is a problem, especially with low molecular weight gases. Figure 7 shows the cause of such leakage. With conventional flat-faced valves, very low unit sealing pressures are available because of the enormous s e a t i n g areas and the small differential sealing force. However, a line-type s e a t (Figure 7) allows increased unit sealing pressure a t a valve and thus complete s e a l i n g . Constant p o p ping of this type valve will, of course, necessitate remachining, but fortunately .I thh would be a minor in operation. Figure 8 shows a method Figure 8. Excess flow of converting a standard check valve in connector high pressure c o n n e c t o r into a velocity check valve. Such valves are useful for preventing the flow of gas into an operating area when a gage or flowmeter ruptures. During normal operating periods, the poppet remains a t the bottom of the valve. On failure, it rises and seats instantaneously, because of a large static pressure drop created across the seat. Figure 9 is a schematic arrangement of the pilot plant and bench scale high pressure area a t the Bruceton, Pa., laboratories. Open-topped cubicles, 1 foot thick, are located behind a curtain wall, which runs from the ground level to the top of the building. These cubicles are used to encase equipment operating a t elevated temperatures, whereas the intervening areas are used for e q u i p ment operating at modest temperatures. Valves are actuated and instruments are observed from a distance, because personnel are not permitted within this area. The laboratory is ventilated by both positive- and induction-type blowers, designed to provide five to ten air changes per hour. As shown in Figure 9, the two gaseous areas are maintained under a slightly negative pressure, while the control area is supplied with a slightly positive pressure to blanket operating personnel with fresh air in the event of gas leakage. RECZWED for review February 16,1956.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACOEPTED March 27, 1956.
837
