Module 3

Surge Phenomena

Surge Phenomena and How It Happens

Parameters That Causes Compressor Surges

Factors Affecting the Compressor Performance Curves

Surge phenomena and how it happens

Figure 3-1 shows the compressor characteristics of, an air compressor, running at constant speed with a certain inlet pressure and temperature. The compressor is supplying air to a receiver.The load curve, which is the system pressure drop (Loss) between the compressor discharge and the air receiver versus the flow, is normally an exponential curve. When there is no flow the system pressure drop should be zero and as the flow increase, the pressure drop increases.

Figure 3-1 Compressor characteristics and load curve

The intersection point of the load curve and the characteristic curve defines the operating point. It should be noticed that as the resistance of the downstream system (i.e. downstream the compressor) increases, the load curve shifts upward to intersect with the compressor characteristic curve at the new operating point.

Let us suppose that the air consumption is decreased to Q2; then the back pressure in the receiver will be higher and consequently the downstream system resistance will

increase causing the load curve to shift upward (load curve"2"). When the air consumption is decreased further, we eventually arrive at Q3.The point of intersection of the compressor curve and the load curve (point T) represents the maximum pressure that could be delivered by the compressor.

With further decrease in air consumption, the compressor curve goes down, and as the back pressure in the receiver becomes higher than the output pressure of the compressor, the direction of the air flow through the compressor, will be reversed until the output pressure again exceeds the receiver back pressure.

This phenomenon of rapid pressure and flow pulsation is called surging, pumping or cycling and it can vary in intensity from an audible noise to a violent shock. Severe or violent surges are capable of causing complete destruction of components in the compressor such as blades and seals.

In order to prevent this phenomenon from happening, the flow through a compressor should never be reduced to the extent that the compressor starts surging.

Figure 3-1 shows the surge point for one particular speed of the compressor (point T on the curve), but as a matter of fact, if the impeller speed changes the surge point will also change.

As there is one surge point for every speed of a particular compressor, a curve can be drawn through these points which is usually called the surge curve or the surge limit as shown in Figure 3-2

Figure 3-2 surge curve

Choke or stonewall

The surge point represents the low flow limit of compressor operation. The high flow, low efficiency area of compressor operation is referred to as "choke" or "stonewall ".

Compressor surge

These terms are commonly used to describe a maximum flow condition (or a reciprocating compressor. It is a condition in which a flow passage in the compressor is too small to allow more gas to flow regardless the amount of power added. A centrifugal compressor does not normally encounter this situation. All compressors are sized so that flow is limited by maximum speed or by maximum horsepower. No flow passages are so small that they choke the flow. The two terms are, however used to describe the high flow, low efficiency region of the performance curve.

Compressors are normally staged so that they don't operate in this region. Some of the design operating points will however occasionally fall into this undesirable part of the curve. No mechanical damage can result from continuous operation in choke or stonewall.

Compressor surge

As explained earlier, the peak of the speed line represents the surge point at that speed for a centrifugal compressor. This point is the low-flow limit of compressor operation. To the left of the surge point, the compressor impeller is not able to produce enough head to overcome the ratio imposed upon it. There is a breakdown in the flow passing through the machine. Back pressure in the system cannot be overcome by the head produced, and flow reversal occurs.

Figure 3-3 surge limit lineThe head cfm maps in previous figures have shown only the stable operating area for the compressor. A head/cfm map that shows the performance of the compressor to the left of the surge limit point is included in Figure 3-4.

Figure 3-4 performance to the left of the surge limit point

To the left of the surge limit point, compressor headmaking capability decreases as flow decreases. The reason for this can be understood if we look at gas flow through the compressor impeller and diffuser.

Figure 3-5 shows a compressor impeller with the forward shroud removed exposing the blades. The flow enters the Impeller at the eye and turns 90 degrees to continue radially in the flow passage toward the outer diameter.

Three different flow conditions are shown in the passages.

During normal operation, in the area below maximum speed and to the right of surge, a continuous film of gas attaches to the blades on each side of the impeller flow passage. This film is called the boundary layer and its condition generally controls the compressor operation. With the boundary layer continuous throughout the passage, the entire diameter of the impeller is effectively used. At the flow it reduced or the ratio is increased until the operation is at or near the surge line, the boundary layer near the impeller tip begins to become unstable. As a result, the effective diameter begins to

change as this Instability continues. When the boundary layer near the Impeller tip breaks: down, the effective diameter is reduced.

Figure 3-5 boundary layer breakdown

Remember that head is proportional to Impeller diameter. When the boundary layer breaks down at the impeller tip, less of the impeller surface is used to accelerate the gas. This lowers the impeller head making capability.

The Impeller is no longer able to add enough energy to the gas to equal the pressure in the diffuser at the tip of the Impeller. As a result, the gas in the diffuser flows back Into the Impeller until the local diffuser pressure is equal to the pressure of the gas leaving the impeller. The boundary layer reattaches and the pressure builds until the boundary layer once again breaks down. This is a mild surge condition and will continually repeat until the flow is increased r the pressure ratio is reduced. When the flow to the compressor is reduced to the left of the surge line, a violent surge condition occurs in which the entire boundary layer on each of the flow passage breaks down. When this occurs, the gas flows back into the impeller from the diffuser but much more violently than in the mild surge condition. When the pressure in the diffuser is reduced to a low enough level so that the boundary layer can be once more established in the Impeller, the pressure will once again build until the boundary layer again breaks down.

The phenomenon of surge throughout the system can be further explained by analyzing the compressor actual characteristic of pressure versus flow Figure 3-6.

Figure 3-6 system surge

Let us suppose a compressor is running at constant speed on a certain point (A) on the characteristic curve of pressure versus capacity. If the flow is reduced to (B), which is accomplished by sudden increased downstream resistance, the compressor will keep running in a stable manner because, although there is a pressure increase in the discharge pipeline due to the resistance, the pressure produced by the compressor is also higher on (B) than on (A) and can overcome the resistance.

When the compressor is operating on point (C), which is the peak of the curve, any momentary increase in the downstream resistance reduces the flow through the compressor to a region where the compressor produces less pressure than before. Thus, the unit is unable to compensate for the increased discharge pressure. The flow momentarily reverses its direction, taking the operating point to zero flow (point D), which relieves the discharge pipeline, reducing its pressure. The gas then returns towards the compressor discharge with a flow (E) corresponding to the pressure of point (D). But that flow is excessive for the resistance in the pipeline, and the operating point moves towards (C) and all the oscillation cycle is repeated.

Surge symptoms

A surge condition can be characterized by loud clattering noise from the compressor, successive slamming of the discharge check valve, great increase in discharge temperature, rapid pulsation of flow and discharge pressure, and eventual shutdown of the machine either by the operator’s hand or automatically from vibration or high temperature.

Effect of surge

Surge can be mild and almost undetectable except for decreased performance efficiency and the associated Increase in discharge temperature, or It can be violent producing high frequency reversals In the axial thrust on the compressor shaft. Surge can become severe enough to damage the internal parts of the compressor; Operation in surge should always be avoided.

Mild Surge

Surge need not occur simultaneously in all parts of a compressor. For example, because the gas velocity at the walls of a diffuser is less than in the center of the diffuser, as flow is reduced, flow reversals occur first at the walls while the main flow down the middle of the diffuser is still in a forward direction. Also, surge can occur in one passage of an impeller but not another. This partial surge may manifest itself as:

Periodic dips in flow rate, which maybe as short as 0.2 second duration. Pulsation in discharge press especially if measured near the discharge flange or

in the diffuser. Increased vibration levels, both axial and radial. Rise in discharge temperature.

Figure 3-7 flow oscillates at a high frequency or randomly within a narrow flow range, at fairly constant pressure

Small flow oscillation (narrow band) cumulative damage

Violent Surge

Violent surge cannot easily be mistaken for anything else. Typically, flow suddenly fails drastically, the discharge check valve slams, and the unit may immediately trip off-line on vibration and overspeed, or trip after some period of time on high discharge temperature. The compressor may sound like it is swallowing itself. Unless the process condition that brought the compressor to the surge limit is corrected, violent surge is typically repetitive, with a time between surge events on the order of one to several seconds. Surge tends to be repetitive because a surge event drops the discharge pressure, allowing the compressor to restore flow in the positive direction. Restoration of forward flow results in discharge pressure rising and flow falling again to the surge point.

Figure 3-8 Theoretical model for violent surge

D - A Pressure rises and flow falls, because process flow acceptance is less than the surge limitA - B Flow suddenly reverses at constant pressure and the path A - C is not available to the compressor.B - C Flow is negative; pressure falls, until at C the compressor is able to restore forward flow.C - D Flow is suddenly restored.

Field tests over several years showed that low ratio compressors (for example, P2/P1 = 1.2/1) may display a fairly broad flow range of mild surge prior to reaching total flow reversal, perhaps 10%. However, high ratio compressors (for example, P2/P1 = 9/1) may have a much narrower mild surge range. For example, tests on a gas-lift compressor in the Middle East (P1 = 300 kPa/45 psia; P2 = 6800 kPa/1000 psia) showed practically no difference between the flow value where mild surge was noticeable and the flow value were complete flow reversal occurred. When flow was reduced to the surge limit, the compressor essentially snapped directly into violent surge..

Figure 3-9 Example time plot for mild and violent surge flows for a situation where station flow falls slowly to surge, then slowly rises after surge

Process Conditions That Can Lead to Surge

Rise in Discharge Pressure

If suction pressure (P1) and RPM are held constant, a rise in discharge pressure (P2) a fall in flow, ultimately reaching the surge limits. The process condition that causes discharge pressure to rise is a decline in flow-acceptance of the load.Examples are:

Booster and gathering compressors: Flow taken from a pipeline by consumers being sustained at a rate less than flow

put into the pipeline by sources.

Process compressors: Loss of a process train downstream of the compressor. Increased discharge throttling..

Gas Bit compressors: Gas-lift wells shut-in or loaded up.

Compressors operated in series: Trip of a downstream compressor.

Fall in Suction Pressure

If discharge pressure (P2) and RPM are held constant, a fall in suction pressure (results in a fall in flow, ultimately reaching the surge limit. Examples of process situations that can cause this are:Booster and gathering compressors:

Gas flow from a source less than the flow rate of the compressors drawing from the source

Process compressors: Loss of a process train upstream of the compressor. Increased suction throttling.

Compressors operated in series: Loss of an upstream compressor.

Fall in RPM

If suction and discharge pressures are held constant, a fall in RPM can vary quickly drop the flow to the surge limit. In fact, sudden reduction in RPM is the most serious upset that a surge control system has to handle. By comparison, suddenly closing the discharge or suction valve is usually an easier upset to control than sudden RPM decline.

Other Process Changes

Drop in m mass, also called awl weight or specific gravity (SG). Rise in suction temperature.

Changes to Equipment

Compressor impellers worn by sand and other abrasives. Internal gas leakage, for example, between stages or across the balance piston

labyrinth seals. Movement of inlet guide vanes. Plugging of inlet strainer or discharge gas cooler (hydrates). Increased interstage condensate dropout (reduces volume flow to final stages). Deposits of solids or tars on the impellers. Compressors taking source gas from

very low pressure crude separators are prone to this, as are compressors where the process gas contains asphaltines.

Parameters That Causes Compressor Surges

The following parameters could directly cause the compressor surging phenomena as shown in Figure 3-10

A) Low flowB) Low suction pressureC) Low suction temperatureD) High discharge pressureE) High discharge temperatureF) Molecular weight changes

Figure 3-10 different parameters affecting surge curve

Factors Affecting the Compressor Performance Curves

Referring to Figure 3-10 and assume that for the design conditions, point a represents the normal operating point for a constant speed and constant inlet gas conditions. In this case, the flow through the compressor could be reduced to point B without the danger of surging.

Inlet Temperature Effect

When at this flow capacity (speed remains constant) the gas inlet temperature decreases to say, 83% of the design value, the effective compressor characteristics moves upwards and the compressor will enter into the surge region, (point D).

Inlet Gas Molecular Weight Effect

It can also be seen that when the molecular weight of the gas increases the compressor characteristic curve will consequently moves up. It is therefore clear that in the compressor control design, account must be taken of the changes in gas inlet conditions.

Suction Pressure

Similarly, the decrease in suction pressure will move the compressor characteristic curve downwards, thus the operating point will become closer to the surge curve.

Performance Maps and the Surge Limit Line

A compressor PERFORMANCE MAP shows the expected “head” across a compressor for given RPM and “flow” values, holding gas composition and temperature constant. Many different variables are used as “head” variables, such as:

Adiabatic (isentropic) head. Polytropic head (compressor designers prefer this although polytropic head is

more difficult for users to employ in process calculations). Isothermal head (especially for multi-stage intercooled compressors). P2/P1 (can be directly measured; easily derived from adiabatic head). P1JP2 (can be directly measured; easily derived from adiabatic head). P2 (if P1 is constant). P1 (if P2 is constant).

Likewise many different variables are used as “flow” variables: QSTD (standard flow). Q(VOL)1 (suction volumetric flow: actual inlet flow in m3/sec, m3/min, ACFM,

etc.). OMASS (mass flow: kg/sec, kg/mm, lbm/min). QSTD/P1 (useful for determining off-design performance; easily derived from

Q(VOL)1). QSTD/P2 (useful for determining off-design performance; easily derived from

O(VOL)1).

Each map typically lists the gas conditions under which the map is valid.

A performance map typically shows several RPM lines plotted against the Y axis (“head”) and X axis (“flow”). With declining flow, RPM lines end at a line called the surge limit line. This line represents the lower flow limit where MILD SURGE is expected to start, and may include a safety factor added by the manufacturer. Some maps show two lines: a “limit of stable operation,” where mild surge starts, and a “surge limit” where violent surge is expected to start.

The most useful maps for consideration of surge control are the P2/P1 vs. OSTD/P1 map for operation at constant suction pressure, and P1/P2 vs. OSTD/P2 map for operation at constant discharge pressure. These maps are easily derived from HEAD vs. Q(VOL)1 maps.

Figure 3-11 typical performance map representing operation at constant suction pressure.

Some conclusions can be drawn from the above map: At a fixed RPM as flow falls, flow rate becomes increasingly sensitive to pressure

changes. At high flow rates, reducing the pressure has hardly any effect on flow (the unit is

said to be at the stonewall limit). The surge limit line appears to approximately follow a square root curve:

That is at surge:

Equation 3-1:

Which is, at constant gas conditions, equivalent to:

Equation 3-2: Q(VOL) = C1

That the surge limit line is nearly a square root curve is important because it leads to simple surge control system for low ratio compressors.

The simplest “proof’ of equation 3-1 for a particular compressor is to convert the particular map units to P2/Fl vs. Q(STD)/P1 units and plot a curve using equation 1 selecting C so that the curve goes through one point on the surge limit line. For low ratio compressors, the surge limit curve will usually follow equation 3-1 quite closely.

Surge Limit for High Ratio Compressors

Unfortunately, with compressors of increasing ratio, the actual surge limit line deviates increasingly from a curve approximated by equation 3-1, which makes surge control more difficult.

As ratio rises above 1.5/1, and especially if the compressor is intercooled, the above square root approximation gets worse. In such compressors, the surge limit line tends to bend to the right, and surges at higher ratios occur at increasingly higher flows than predicted by equation 3-1 (see Figure 3-12). Two reasons for the higher surge flow point are changes in compressibility of the gas in the final impeller stages due to increasing pressure and temperature, and (in intercooled compressors) decrease in volumetric flow in stages after the intercooling due to increased gas actual density due to the intercooling.

Figure 3-12 typical performance map for high ratio compressor. Note deviation from square root curve as ratio rises

Conclusions about Surge by Inspection of Performance Maps

Careful Inspection of a large number of performance maps which have been produced holding various parameters constant will lead to additional conclusions:

Figure 3-13 P1: constant

At fixed suction pressure, a non-intercooled compressor’s surge point flow rises with increased discharge pressure.

Figure 3-14 P2: constant

At fixed discharge pressure, a non-Intercooled compressor’s surge point flow is dependent on suction pressure, but to considerably less extent than the dependence on discharge pressure.

Figure 3-15 P1: constant (intercooled)

With Intercooled compressors operating at fixed suction pressure, the surge point flow generally follows s curve that is similar in shape at low ratio to that of a non-Intercooled compressor, but et higher ratios the surge point flow Is considerably higher than would be expected by extrapolating a low ratio non-Intercooled compressors surge limit curve. On a performance map, the surge limit line bends to the right with increasing ratio. Clearly, the shape and location of the composite surge limit line of all the stages vary strongly with ambient temperature (cooling) and Interstage drop out (Inlet composition, cooling).

Figure 3-16 P2: constant (intercooled)

With Intercooled compressors operating at fixed discharge pressures, the surge point flow generally Increases as suction pressure rises or falls from design suction pressure. The surge margin Is greatest at approximately design suction pressure. Thus, at fixed flow raising the suction pressure can cause the compressor to surge or recycle!