Mechanical Maintenance Training

Course title: Compressors

What is a compressor?

A compressor is a mechanical device that produces flow and/or pressure in a fluid by the expenditure of work. Usually used to handle large volumes of gas at pressure increases from 10.32KPa to several hundred KPa.

Types of Compressors

Continuous-flow compressors (operate by accelerating the gas and converting the energy to pressure)• Centrifugal• Axial flow

Positive Displacement compressors (operate by trapping a specific volume of gas and forcing it into a smaller volume)• Rotary• Reciprocating

Compressor SelectionCentrifugal – Used for medium to high pressure delivery and medium flow

Axial Flow – Used for low pressure and high flow

Positive Displacement - Used for high pressure and low flow characteristics

Compressor SelectionFactors to be considered:1.Flowrate2.Head or pressure3.Temperature Limitations4.Method of Sealing5.Method of Lubrication6.Power Consumption 7.Serviceability 8.Cost

Reciprocating Compressor

Principles, Construction & Design Philosophies

BASIC COMPONENTSCONNECTING ROD

CROSSHEAD

PISTON

VALVE

PISTON ROD

PISTON ROD PACKING

PISTON CYLINDERCRANKSHAFT

CRANKCASE

Pressure-Volume DiagramThe P-V diagram (pressure-volume diagram) is a plot of the pressure inside the compression chamber (inside the bore) versus the volume of gas inside the chamber. A complete circuit around the diagram represents one revolution of the crankshaft. This is an “ideal” diagram in that it does not show any valve pressure and therefore no valve loss horsepower. PD is discharge pressure (typically said to be the pressure that exists at the cylinder flange). PS is suction pressure.

THEORY OF OPERATION:

This depicts the compression event. It starts at the point where the suction valve closes. When the suction valve closes, gas is trapped inside the compression chamber at suction pressure and suction temperature. As the piston moves towards the other end of the compression chamber, the volume is decreasing, the pressure increasing and the temperature increasing. Compression stops when the discharge valve opens. The shape of the curve of the compression event is determined by the adiabatic exponent (k-value or n-value).

Compression

When the discharge valve opens, compression stops, and gas at discharge pressure and discharge temperature is pushed out of the compression chamber through the discharge valve, into the discharge gas passage and out into the discharge piping. The discharge event continues until the piston reaches the end of the stroke, where the discharge valve closes and the next event, expansion, begins. The compression and discharge events together represent one-half of one revolution of the crankshaft and one stroke length.

Discharge

When the discharge valve closes at the end of the discharge event, there is still some gas left in the compression chamber. This volume of gas is referred to as the “fixed clearance volume” and is usually expressed as a percentage. As the piston moves away from the head, the volume inside the compression chamber increases with all of the valves (suction and discharge) closed. The gas in the fixed clearance volume expands, decreasing in pressure and temperature, until the pressure inside the compression chamber reaches suction pressure, where the suction valve opens and the expansion event ceases.

Expansion

Suction

At the end of the expansion event, the suction valve opens opening the compression chamber to the suction gas passage and suction piping system. As the piston moves, the volume in the compression chamber is increasing and the compression chamber fills with gas at suction pressure and suction temperature. The suction event ceases when the piston reaches the other end of the stroke, the suction valves closes and the piston turns around and goes the other direction. The end of the suction event marks the end of one complete cycle. One complete cycle requires one complete revolution of the crankshaft and two stroke lengths.

COMPRESSOR VALVE TYPES:

Valves are key components for the successful operation of a piston compressor. They are the most stressed components of the compressor. Their perfect operation is decisive for the delivery of the gas. According to a study, more than one third of all compressor-related shut-downs are caused by valve problems. The most important valve types are– plate, ring and poppet valves. The common feature of these valves is that they are self-acting, i.e. by means of differential pressure. The principal components are the valve seat, stroke limiter and central bolt together with sealing elements in the form of plates, rings or poppets and their associated spring elements and spacer rings.

COMPRESSOR VALVE TYPES:

1. PLATE

The plate valve is the oldest self-acting design. Concentric rings joined together by radial connections with the appropriate spring constitute the sealing element. Depending on the design, one or more damper plates are employed. Metal or plastic material is used for the valve and damper plates. Plate valves have large flow areas, but they have unfavorable flow characteristics. The gas has to be deflected twice through 90°, which leads to corresponding valve losses.

Open Position Close Position

Valve Seat Body

Seat Plate Valve Spring

Valve Guard

Cross-Sectional View

COMPRESSOR VALVE TYPES:

2. RING

The sealing elements of ring valves comprise single rings that are always made of plastic. Ring valves are among the most flow-effective valves, because gas can flow through the valve with only slight deflections. This leads to lower losses, despite their smaller flow areas. Further advantages of this valve are its simple assembly and the stable form of the sealing elements, which reduces the risk of fracture. A further positive feature is that foreign particles can embed themselves in the plastic material, and so they are more robust than comparable metal-plate valves. Moreover, there is less danger of clogging by condensing gases or gases containing hard particles. The machining of the valve seats during refurbishing of ring valves is even more complex. In addition, plastic is not suitable as a ring material for some gases, and high-temperature plastic rings cost considerably more than metal plates.

COMPRESSOR VALVE TYPES:

3. POPPET Poppets have been used in the earliest valve designs for compressors. Weight and impact forces limited the use of bronze and steel poppets. The modern poppet valve was introduced in the 1950's. It used mushroom shaped sealing elements made of metallic materials or thermoplastics. The poppet material determines the application range of the valves. The use of metallic poppets limits the compressor speed to about 450 rpm. The development of heavy-duty thermoplastic materials like PEEK and their application for sealing elements has extended the range for poppet valves significantly. Compressor speed of up to 1800 rpm, temperatures up to 220°C and differential pressures of 100 bars are no longer a problem. Their characteristics are very similar to those of ring valves. They also have effective flow characteristics, i.e., the losses in the sealing gap are lower than those of plate valves. Poppet valves are less likely to leak at higher temperatures, because geometric distortions and thermal expansion of the poppets do not have any negative effects. One disadvantage, however, is the larger number of sealing elements, with which the failure probability of a single element increases. Nevertheless, this point can also be viewed in a positive manner, because further operation is possible even if individual poppets should fail for a certain period of time.

Centrifugal Compressor

Principles, Construction & Design Philosophies

BASIC COMPONENTS

INLET

IGV

IMPELLER

VOLUTE

THEORY OF OPERATION:

Centrifugal compressors accelerate the velocity of the gases (increases kinetic energy) which is then converted into pressure as the gas flow leaves the volute and enters the discharge pipe.

Centrifugal force is utilized to do the work of the compressor. The gas particles enter the eye of the impeller designated D in the figure shown. As the impeller rotates, air is thrown against the casing of the compressor. The air becomes compressed as more and more air is thrown out to the casing by the impeller blades. The air is pushed along the path designated A, B, and C in the figure. The pressure of the air is increased as it is pushed along this path. Note in the figure that the impeller blades curve forward. Centrifugal compressors can use a variety of blade orientation including forward and backward curves as well as other designs. There may be several stages to a centrifugal compressor and the result is that a higher pressure would be produced.

Figure 1 – Dry Gas Seal Cross-section

PRIMARY SYSTEM SEALING:

Dry gas seals have been applied in process gas centrifugal compressors for over 20 years. Over 80 percent of centrifugal gas compressors manufactured today are equipped with dry gas seals. Dry gas seals are available in a variety of configurations, but the "tandem" style seal (Fig. 1) istypically applied in process gas service. Other types of gas seals (such as double opposed) are not considered. Tandem seals consist of a primary seal and a secondary seal, contained within a single cartridge. During normal operation, the primary seal absorbs the total pressure drop to the user's vent system, and the secondary seal serves as a backup should the primary seal fail. Dry gas seals are basically mechanical face seals, consisting of a mating (rotating) ring and a primary (stationary) ring (Fig. 2). During operation, grooves in the mating ring (Fig. 3) generate a fluid-dynamic force causing the primary ring to separate from the mating ring creating a "running gap" between the two rings. Inboard of the dry gas seal is the inner labyrinth seal, which separates the process gas from the gas seal. A sealing gas is injected between the inner labyrinth seal and the gas seal, providing the working fluid for the running gap and the seal between the atmosphere or flare system and the compressor internal process gas.

Figure 3Figure 2

Equipment Overhauling

The following are the general requirements before overhauling the equipment:

•Make sure the system is purged and evacuated of hydrocarbons.

•Install spades at the necessary blinding points.

•Tools and other lifting devices delivered and installed on site.

•Coordination meeting on the extent of the job to be performed.

•Checklist and other information on clearances are available.

• Rod drop-out/ crosshead clearances• Rod packing, oil scrapers and seals• Deflection and Alignment on Crankshaft• Valve condition• General Clearances and Alignment• Connecting rod/ Piston Rod• Equipment Levelling• Cleanliness

The following are the things to be inspected during the assembly and disassembly process of the equipment:

As a prerequisite the following tests shall also be done in following parts of the equipment:

• Dye Penetrant Testing of Pistons, Crossheads, Valves, Main bearing metal, Cylinder liner and housing (if necessary) • Radiographic Testing on the piston nut and rod threads

RECIPROCATING COMPRESSORS:

• Impeller tip/seal clearances• Drive bearings condition• Dry gas seal condition• General Clearances and Alignment• Equipment Levelling• Cleanliness

The following are the things to be inspected during the assembly and disassembly process of the equipment:

As a prerequisite the following tests shall also be done in following parts of the equipment:

• Dye Penetrant Testing of housing (if necessary)

CENTRIFUGAL COMPRESSORS:

For each particular design of compressor the maintenance and overhauling manual should be provided by the manufacturer. This should be the main reference of the maintenance technician when doing the maintenance. All of these are available in the library. The technician should familiarize himself with all the details necessary for the maintenance of the compressor as recommended by the vendor.

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