## Archive for the ‘Fundamental’ Category

## DYNAMIC COMPRESSORS – SURGING

When a compressor is connected to a large system, which needs a large amount of gas for the process, the gas intake will vary with the process requirement. During startup, the gas demand will be very high. Hence the compressor will run at its full capacity.

As the process proceeds, the demand for gas will start reducing. If the system does not use the gas as quickly as the compressor delivers, the system pressure will increase.

With the increase in the system pressure, the resistance to discharge of the compressor increases.

Due to this, the compressor capacity reduces.

When the head required maintaining the flow increases above the maximum head of the compressor, the gas flow stops.

Under this condition, the pressure within the compressor becomes less than the system pressure. This results in the gas flow from the system to the compressor, called “flow reversal.”

When some quantity of gas has gone to the compressor section, the system pressure will drop.

When the system pressure becomes less than the maximum head of the compressor, the compressor starts delivering the gas to the system.

The compressor operates at a lower capacity and higher head/pressure.

If the system pressure system builds up again to a high pressure, the flow reversal will repeat. The cycle continues.

The rapid flow of gas back and forth the compressor is called surging.

Surging occurs when the compressor operates below the minimum capacity.

The rapid reversals of surging set up severe vibrations in the compressor and piping, which may cause damage to the compressor.

A compressor goes into surging because the flow of gas drops down below the minimum stable limit.

Most compressors are protected against surging by anti-surge control methods as explained below.

Refer to the figure shown below.

Y axis shows the percentage of rated head and x axis shows the percentage of rated capacity of the compressor. A point higher on the graph shows higher head. The compressor capacity varies between 40 to 110 percent. The curve shows, as the flow increases, head decreases.

The compressor is discharging into a system that requires 100 percent

of its rated head. According to the graph, the compressor is operating at

100 percent of its capacity.

Suppose the discharge system does not use as much gas as the compressor delivers the pressure at the discharge end of the compressor increases as may be seen from the graph.

When the gas flow reduces to 90 percent, the head developed increases to 102 percent of the rated head. As the gas discharged from the compressor reduces, the pressure developed by the compressor increases. When the flow reduces to 50 percent of the rated flow/capacity, the compressor no longer delivers the gas to the system and virtually the gas flow stops.

When the compressor pressure becomes lower than the system pressure, the gas will flow from the system to the compressor (flow reversal).

When this condition is reached, the system pressure will start dropping down, and the compressor will discharge gas to the system again.

This repeated process of forward and reverse flow is called “surging.”

## DYNAMIC COMPRESSORS – RPM AND BREAK HORSEPOWER

The ratio of compression, defined as the ratio of the discharge absolute pressure to the absolute suction pressure, is an indicator of the amount of pressure that the compressor adds to a gas or gas mixture. At any particular RPM, a dynamic compressor adds certain head (pressure) to the gas. The total head/pressure developed depends on the compressor design, its RPM, and the amount of gas flow at suction. At a fixed RPM and inlet CFM of gas, the head developed by a centrifugal compressor is the same irrespective of the weight of gas. The head developed by a centrifugal compressor does not depend on the density of the gas and it is possible to convert the feet of head into psi (pounds per square inch) and vice versa.

The density of a gas does not affect the head developed, but affects the discharge pressure of the compressor.

Example:

Two identical compressors handle 200 cfm of air and hydrogen at 12,000 rpm and as per the compressor characteristics; the differential head developed is 20,000 feet at discharge conditions. What will be the discharge pressure under these conditions? (Densities at discharge conditions for air and hydrogen are 0.65 and 0.15 lb/ft3, respectively.)

Case 1.

Discharge head for air = 20,000 ft: Density = 0.65 lb/ft3

Discharge pressure = head × density = 20,000 × 0.65 lb/ft2

= 13,000 lb/ft2

= 90.30 lb/in.2 = 6.35 kg/cm2 g

Case 2.

Discharge head for hydrogen = 20,000 ft: Density = 0.15 lb/ft 3

Discharge pressure = head x density = 20,000 × 0.15 lb/ft2

= 3,000 lb/ft2

= 20.83 lb/in.2 = 1.46 kg/cm2g

From the above example, it is clear that for the same CFM flow, RPM and the head developed the discharge pressure is high for a high-density gas than for a low-density gas.

BHP refers to the break horsepower that is required by the compressor shaft to achieve the desired compression ratio. Because the gas density fluctuates very often in industrial systems, a centrifugal/dynamic compressor tends to change power consumption while in operation.

The ratio of compression, R, is the absolute discharge pressure divided by the absolute suction pressure. R is an indicator of the amount of pressure that the compressor adds to the gas.

At a given RPM, a dynamic compressor adds a certain head to the gas.

The total head added depends on:

The design of the compressor

The amount of gas flow and

The operating RPM (speed)

As RPM increases, the total head developed by the compressor increases.

At a fixed RPM and CFM, the compressor attains approximately the same feet of head, regardless of the weight of the gas handled.

The head developed by the compressor does not depend on the density of the gas being handled.

The feet of head or the meter of head can be converted into PSI or kg/cm2 equivalent.

Similarly, PSI or kg/cm2 can be converted into feet or meters, respectively.

When a compressor at a given RPM is handling a heavier gas, the work it does over a pound of gas is the same as the work done on a pound of lighter gas.

Head represents the amount of foot pounds or kilogram meter of work done per unit weight.

A compressor at a given RPM handles two different gases. The gas that requires a larger volume per unit weight is the lighter gas.

## DYNAMIC COMPRESSORS – RPM AND HORSEPOWER

RPM AND HORSEPOWER

For all rotating machines, RPM is an important parameter. The impeller of a centrifugal compressor has to rotate (revolve) to move the gas.

As the RPM of the impeller increases, the velocity of the gas also increases.

Gas velocity is proportional to the RPM of the impeller, as shown below.

For any given RPM, a set amount of work in foot pounds or kilogram meter is done on the gas per unit weight. Since this velocity is converted into head or pressure, for a constant RPM, the head developed is fairly constant.

Whether the gas is heavier or lighter, the work done per pound of gas is the same for the same RPM.

At a given RPM, the ACFM of gas the compressor compresses will be constant. But the weight of the gas compressed will be more, in the case of a heavier gas. When more weight of gas is compressed, the work done on a heavier gas will be more.

When the rate of work done on a gas increases, the horsepower required to compress a heavier gas also increases.

## DYNAMIC COMPRESSORS – AXIAL COMPRESSORS

Motion along the axis of a shaft is called axial motion. This takes place in a straight line. A compressor in which the gas moves parallel to the axis of its shaft is called an axial compressor. An axial compressor has stator and rotor blades, as shown below.

The rotor blades are attached to the shaft and rotate with the rotary motion of the shaft. The stator blades are attached to the casing, as shown in the above figure.

The arrangement of the blades is such that there is a set of stator blades between each two sets of rotor blades, as shown in the figure below.

The rotor blades behave in the same manner as the blades of a fan. As they rotate, they force the gas to move. The rotor blades impart both pressure and velocity to the gas.

The rotor blades force the gas into the stator blades.

As the gas is thrust into the stator blades, the openings between the blades act as diffusers and reduce the velocity of the gas. With the decrease in the velocity, the pressure of the gas increases. The stator blades guide the gas into the next set of rotor blades. The gas entering the second set of rotor blades has a slightly higher pressure. Thus, each set of stator and rotor blades increases the gas pressure.

In axial compressor, the pressure increase of a gas is achieved by using many sets of stator and rotor blades. The blades in an axial compressor are not of the same size. The blades get gradually smaller toward the discharge end of the compressor, as shown below.

As the gas flows through an axial compressor, it occupies less volume successively in its flow path. Thus, the gas pressure increases. The flow of gas in an axial compressor is somewhat linear and in the direction

of its axis.

Two forms of gas energy are pressure and velocity. Energy cannot be created or destroyed, but it is convertible from one form to another. By doing work on a gas, the compressor adds energy to the gas. The total energy of a flowing gas is a function of its pressure, velocity, and temperature. Where the gas velocity reduces, pressure increases (Bernoulli’s Theorem).

## HEAD OF COMPRESSION

For compressing a gas to higher pressures, certain amount of work has to be done on the gas. The work done on a gas may be expressed as foot pounds or kilogram meters.

When 1 lb of gas is lifted or moved to a distance of 1 ft, then the work done by the compressor is 1 ft·lb. Similarly, when 10 lb of gas is moved to a distance of 1 ft, then the work done by the compressor is 10 ft·lb. When 1 lb of gas is lifted/moved to a distance of 10 feet, then also the work done by the compressor is 10 ft·lb.

The head developed by a compressor is the distance or height to which a column of gas can be moved at the average density of gas. This is an important factor in calculating the head developed by a compressor. For each pound of gas the compressor raises the head, a corresponding amount of work in foot pounds has to be done on the gas. As shown above, if the head increases, the number of foot pounds of work to be done on the gas per pound also increases.

Pressure may also be converted to head, as shown in the example below.

Example:

1,000 cfm of air is compressed by a compressor to a pressure of 6 kg/cm2g

pressure. What is the work required to be done by the compressor on the

air and what is the head developed?

Air inlet pressure = atmospheric pressure = 14.7 psia

Air outlet pressure = 6 × 14.22 + 14.7 psia = 100 psia

Air density at atmospheric pressure = 28.84/359 lb/ft3 = .0803 lb/ft3

Weight flow rate of air = 1,000 × 0.0803 lb/min = 80.3

Air density at 100 psia = 28.84/50.19 = 0.5746 lb/ft3

Average density of air = (0.0803 + 0.5746)/2 = 0.3274 lb/ft3

Differential pressure developed by the compressor = 6 × 14.22 psi

= 85.32 psia

= 12,286.08 lb/ft2

Dividing the above value by the average air density, we get

= 12,286.08/0.3277 ft

= 37,491.8 ft

Work done on 1,000 cfm air = 80.3 × 37,491.8 ft·lb/min

This is the work to be done by the compressor on 1,000 cfm of air.

## COMPRESSOR CAPACITY

The capacity of a compressor is the volume of gas that it handles in a given period of time. For example, CFM indicates the volume of the gas handled by the compressor in 1 minute. The flow rate of a gas in CFM in a pipe line depends on the velocity of the gas and the diameter of the flow path. For the same velocity, the rate in CFM is higher for a larger diameter passage.

Example:

Air passes through two pipe lines, one with 6 in. diameter and another with

8 in. diameter. The velocity of air in both the pipes is 500 ft/min. What is

the flow rate of air through these two pipe lines?

Denote the diameter of the 6 in. pipe as d1 and that of the 8 in. pipe

by d2.

a.b.c.d.Area of cross-section of the 6 in. pipe = ? × d12/4 = (22/7) × (6/12)2/4

= 0.19643 ft2

Volume of air flow =

velocity × cross-section area = 500 × 19643

= 98.2 cfm

Area of cross-section of the 8 in. pipe = ? × d22/4 = (22/7) × (8/12)2/4

= 0.34921 ft2

Volume of air flow =

velocity × cross-section area = 500 × 0.34921

= 174.6 cfm

Result: Air flow rate in 6 in. and 8 in. diameter pipes for the velocity

of 500 ft/min are 98.2 and 174.6 cfm, respectively.

If the gas velocity is greater, then the pressure at the discharge section

is lower.

During compression, the volume of gas entering the compressor is

greater than the gas leaving the discharge. The ACFM is measured at the

suction of the compressor.

## DYNAMIC COMPRESSORS – RATIO OF COMPRESSION

While the difference between suction and discharge pressures denotes the work done on a gas system, the ratio of absolute discharge pressure to absolute suction pressure is known as compression ratio. When a gas is compressed, part of the energy input or work done is converted into heat and friction losses. The ratio of compression, R, is the relation between the absolute discharge pressure and the absolute suction pressure. If P2 is the discharge pressure and P 1 is the suction pressure, then the compression ratio R = P 2/P1. This means the compression ratio denotes how many times the discharge pressure is greater than the suction pressure. In determining compression ratio, only absolute pressures must be used. To get absolute pressure, add the atmospheric pressure to gage pressure.

For example, compressor discharge pressure = 300 psig: Absolute discharge pressure = 300 + 14.7 = 314.7 psia.

Example:

Air is compressed to 100 psig using an air compressor. What is the com-

pression ratio of the compressor?

Air enters at atmospheric pressure. Therefore, P1 = 14.7 psia

Discharge pressure = 100 psig = 100 + 14.7 = 114.7 psia

Therefore, compression ratio R = 114.7/14.7 = 7.81

Example (metric units):

Air is compressed to 6 kg/cm2g using an air compressor. What is the com-

pression ratio of the compressor?

Air enters at atmospheric pressure. Therefore, P1 = 1.033 kg/cm2a

Discharge pressure = 6 kg/cm2 g. Absolute pressure = 6 + 1.033 = 7.033

Compression ratio R = 7.033/1.033 = 6.81

Note how the pressures have been converted into absolute pressures.

## DYNAMIC COMPRESSORS – CENTRIFUGAL COMPRESSORS

If a body is set in motion, it tends to continue in motion unless some force acts in the opposite direction to stop it. If there is no gravity pull, nor any obstacle to deflect it, a body in motion travels in a straight line.

Suppose a ball attached to a string is set in motion, as shown in Figure 1.9. Assume that there is no gravity and that the string has no effect on the ball. The ball moves in a straight line.

Suppose the string is fastened to a fixed pivot point and then the ball is set in motion, as shown in Figure 1.10. At first, the ball moves in the direction of motion. When the string becomes taut, it deflects the ball. Because of the deflection, the ball actually travels in an arc, or a circle. Assuming it has enough energy, the ball continues to move in a series of arcs. At each instant of its travel, the physical tendency of the ball is to travel in a straight line. But instead, the ball travels in a circle because it is held or deflected by the string.

The string actually applies centripetal (pulling-in-toward-the center) force, causing the path of the ball to change or curve. If the string breaks, the ball flies out in a straight line. Any object traveling in a circle is kept in that path of travel by a force called centripetal force. The force holding the ball in the circle of motion, that is, from the ball to the pivot point is the centripetal force.

To hold the ball in its position and path, an opposite force is required. That force is called the centrifugal force. If the centripetal force is eliminated, the object then moves in a straight line.

The force pulling an object in a circular path toward the center is centripetal force. The centrifugal tendency of the object is its tendency to pull away from the center of rotation, or to pull against the centripetal force. The centrifugal tendency acts in the direction opposite to the centripetal force.

The centrifugal tendency is actually not a force but is the result of the tendency of the object to move in a straight line while being pulled toward a center of rotation by the centripetal force. Assume a ball bearing is placed close to the center of a disk that has blades, as shown in the figure below. As the disk begins to move, one of the blades forces the ball bearing to move. The ball bearing tends to travel in a straight path. The drawing shows the actual path of the ball bearing as the disk rotates.

When the disk rotates, the bearing is forced away from the center of the disk, as shown in the figure below.

Let us consider two points A and B located on the disk. Point A is at the tip of the disk, while point B is closer to the center of the disk. When this disk starts rotating, as shown in the figure above, point A covers a larger distance than point B. When the disk is rotating, point A moves faster than point B. Anything that is being carried along by the rotation of the disk has a greater velocity when it is near the outer rim of the disk. If anything being carried along by the rotation of the disk also travels outward from the center to the outer rim, it gains velocity.

The velocity of point A is proportional to the RPM (revolution per minute), or the rotating speed of the disk. The energy picked up by the material at point A is given by

KE = m × v2/2gc

If D is the diameter of the disk in meters and N is the RPM of the disk, the longitudinal velocity is equal to ? × D × N/60 m/sec (i.e.,3.28 × ? × D × N/60 = 0.17181 × D × N m/sec. The kinetic energy gained will be m × v2/2gc)

To achieve this kinetic energy, work has to be done on the disk or impeller.

The figure given below is a compressor impeller. An impeller is made of two plates separated by blades.

When the impeller begins to rotate, the blades force the air in the impeller to move. Air molecules tend to travel in a straight line. Because there is no centripetal force, the rotation forces the air molecules outward from the center, or eye, of the impeller. As the air molecules move outward, they gain velocity, or speed. The air also tends to oppose the push of the blades, so the pressure of the air is increased. The impeller adds both pressure and velocity to the air.

The tendency of air or gas to move outward from the center of a rotating impeller is the centrifugal tendency. A compressor that uses centrifugal tendency to impart pressure and velocity to a gas is called a centrifugal compressor.

The part of the centrifugal compressor that moves the gas is the impeller. As the impeller rotates, it moves the gas toward its outer rim. As the gas moves toward the outer rim of the impeller, its velocity increases.

This increase in velocity away from the eye creates a low-pressure area at the eye. This low-pressure area causes a suction, which allows more gas to enter. The impeller does work on the gas. The work is converted into the energy that the gas gains, which is in the form of both pressure and velocity. When the gas is at the tips of the impeller blades, it is at maximum velocity. As the gas leaves the impeller, it is thrust into a passageway called the diffuser (refer figures given below). When the gas enters the diffuser, the impeller is not acting directly on the gas.

The radius of the diffuser is larger than the radius of the impeller. Due to the larger radius, the flow path of the gas through the diffuser is in a larger spiral. Since the flow path is longer and there is no direct action by the impeller blades, the velocity of the gas decreases. As the velocity of the gas decreases, its pressure increases.

The diffuser converts the velocity of the gas to increased gas pressure. Gas passes from the diffuser into the volute. In the volute, the conversion from velocity to pressure continues.

Gas passes from the diffuser into the volute as shown below (single stage/last stage of a multistage compressor).

In the volute, the conversion from velocity to pressure continues. In a centrifugal compressor, work is done on a gas to impart both pressure and velocity.

A centrifugal compressor, by doing work on gas, imparts both pressure and velocity to the gas. Then, the velocity of the gas is converted into pressure within the compressor. Look at the compressor below.

• It has four separate impellers.

• Each impeller and diffuser makes a stage.

• This is a four-stage centrifugal compressor.

As the gas leaves the first impeller, it gains some velocity and pressure. The increased velocity is partially converted into pressure in the diffuser.

As the gas leaves the diffuser, it enters the return passage, which guides it into the eye of the next impeller. When the gas enters the eye of the second impeller, it has greater pressure than when it entered the eye of the first impeller. Each impeller adds to the total energy of the gas. It may be noted that the velocity added by the impeller is converted into pressure energy within the diffuser. When the gas leaves the compressor, its pressure is higher than the inlet pressure. The work done by a compressor is the total energy added to the gas through impellers. A gas leaving the compressor has added energy in the form of pressure and temperature.

## DYNAMIC COMPRESSORS

There are two forms of energy in any system. One is called potential energy and the other is called kinetic energy. For example, compressed gas in a static state exerts its pressure in all directions, as shown in Figure 1.8. When the outlet valve is opened, the gas flows out at very high velocity. Depending on the flow rate, the pressure in the cylinder drops down. In this case, the pressure energy is converted into kinetic energy. This kinetic energy is capable of doing work such as driving a pneumatic wrench, hammer, etc. The higher the pressure, the higher will be the velocity and hence the kinetic energy of the gas leaving the system.

A dynamic compressor adds energy to gas in the same manner that an electric fan does. Consider a fan in operation and note the following points:

1. It is the rotating blades of the fan that force the air to move.

2. Air that is at rest tends to remain at rest.

3. As the fan blades start turning, they push on the air. The stationary air resists the push of the blades.

4. As the air resists the blades, the molecules of the air are brought closer together.

5. When the air molecules are compressed, the volume of the air decreases.

6. As the volume of the air decreases, its pressure increases.

7. The blades of the fan overcome the resistance of the air and thrust the air forward.

8. The faster the blades turn, the faster the air is pushed.

9. The fan, by doing work on the air, actually increases the pressure and velocity of the air.

10. When velocity and pressure are added to a gas, its total energy increases.

11. A dynamic compressor increases the total gas energy by adding pressure and velocity to the gas.

12. The total energy of a gas leaving a compressor is greater than the total energy of the gas entering the compressor.

13. The energy that a gas gains in a compressor is due to the work done on it.

## COMPRESSORS INSTALLATION ENGINEERING

After the compressor unit is selected and a purchase order issued and accepted, the next steps require continued vigilance. This is not simple because many more people in the engineering and supplier organizations now become involved. Follow these guidelines to prevent certain items from being neglected:

1. For all but simple catalog units, prepare a process and instrumentation diagram or an engineering flow diagram for the complete compressor system.

2. Establish the layout requirements including those determined by operator assignment, that is, the number of operators assigned to the compressor during normal operations, during startup only, etc. Provide terminals for remote control if an operator will not be in attendance at all times.

3. If a large, complex compressor is involved, hold meetings with the supplier’s engineering group to establish schedules for the submission, review, and approval of the supplier’s engineering data and drawings, and tentative plans for use of his servicemen during installation and startup.

4. Review the compressor manufacturer’s drawings and those of his suppliers to ensure that quality and performance criteria are being met.

5. Review the torsional-vibration analysis and lateral critical studies completed by the compressor and drive supplier to make certain that no contemplated operating condition will cause the machine to operate at a hazardous speed.

6. Review unpriced supplier orders. (Priced orders would not be made available and are unnecessary.)

7. Review control plans, including startup, normal operation, scheduled and forced shutdowns, protective and safety devices for alarm and shutdown, and the duties to be assigned to the operators.

8. Submit to the supplier, for his comments and review, design bases and installation drawings for foundations, piping, and pipe supports. Such information should include the calculated forces and moments (hot and cold) exerted by the piping on the equipment flanges. Guidelines for allowable values are established by the compressor and turbine suppliers on their outline drawings.

9. Review requirements for shop and field pressure and performance testing. In most applications, test procedures established by the supplier are sufficient. Establish procedures for shop erection and match marking of the prefabricated pipe to be furnished by the compressor supplier. Establish what shop tests are to be witnessed.

10. Establish requirements for such items as operating and maintenance access (cranes, monorails, etc.); noise control within buildings and other enclosures; protection from fumes and dust; winterizing; and piping systems, including drains, vents, and access for field flushing and cleaning.

11. Provide methods so that the installed dimensional accuracy of piping right at the compressor is high, and thus compatible with the level of dimensional exactness required by the machinery. Neglect of this may require field changes in the piping arrangement to secure and maintain the acceptable compressor alignment. Arrange the major piping so that supports can be taken from concrete substructures rather than from elevated steel structures. This is most important in reciprocating compressors because it helps in attenuating vibration.

12. Make provisions for shop inspection during fabrication and assembly, as well as during shop testing.

13. Obtain copies of expediting and inspection reports. Monitor delivery schedule.