Mark Sandberg

Centrifugal compressor operations involving high molecular weight hydrocarbons represent some unique challenges. These operations include vapor recovery compressors gathering low pressure and storage tank gases in upstream operations, condensate and liquid stabilizer overhead flows in gas treating and LNG plants, and wet gas compressors in refinery applications. The gas compositions of these compressor streams contain higher fractions of heavier components from ethane to pentane as compared to typical natural gas. These higher molecular weight flows may be subject to partial condensation of the heavier components under some operating scenarios resulting in a potential reduction in mass and volumetric flow rates and a decrease in the molecular weight of the gas stream. While this may be anticipated during normal operation, it may be problematic during transient operation such as start-up and periods of partial recycle during continuous operation. It is also possible to experience significant gas composition variations during normal operations that can impact stable compressor operation. Additionally, these compression services are often viewed as less critical during design phases, so they are not evaluated as thoroughly as possible and are frequently supplied with constant speed drivers.

FIGURE 1: Typical Compressor System Schematic (Case 1)

The basic configuration of a typical compressor system is shown in 

. The inlet to a compressor section should include an isolation (block) valve and a suction scrubber that removes any entrained liquids from entering the compressor. Flow from the compressor section discharge will generally have a large portion of the heat of compression removed with some type of aftercooler with a check valve and discharge isolation valve downstream. The check valve is intended to prevent reverse flow during process upsets while the isolation valve is often held in the closed position during compressor start-up. A recycle line is provided downstream of the aftercooler to upstream of the suction scrubber to accommodate any required recycle flows during normal operation or upsets such as surge avoidance. The included valve supplies the necessary pressure drop from existing compressor discharge to suction pressure conditions. A single valve for both anti-surge and recycle services is often provided, but this may also be accommodated by separate anti-surge and recycle valves since their response times and position accuracy requirements can be different.

Starting centrifugal compressors is typically accomplished with the anti-surge/recycle valves fully or partially open to avoid going into surge and control the starting power required. These valves may also be needed to recycle a portion or all the flow for extended periods of time when the inlet flow supplied is less than the design minimum (when flow rate falls below the minimum to avoid surge) or when discharge process conditions will not accept flow from the compressor. The flow across the recycle valve(s) is described as a Joule-Thomson (isenthalpic) throttling process with a normal accompanying temperature drop that is related to the pressure drop. In some situations, the temperature drop is sufficient to partially condense some of the gas mixture components and result in a gas-liquid two-phase flow on the compressor suction side of the valve.

It is also possible that the temperature drop provided by the aftercooler results in some partial condensation of gas components at the higher pressure existing at compressor discharge conditions but reduced temperature. It is possible to estimate the potential of a gas mixture to produce some condensate liquids by evaluating the vapor pressure of the different gas components. A plot of vapor pressures for a number of hydrocarbon components is given in 

FIGURE 2: Vapor Pressures for Various Hydrocarbons

These values were derived with the National Institute of Standards and Technology’s (NIST) REFPROP computer application.

At pressures above the curve for a given component and temperature, liquids may be present. While providing an approximate estimation of when liquids may form, the vapor pressures are only valid for ideal gas conditions due to molecular interactions between the various gas components and other real gas effects in a gas mixture. Nevertheless, reference to such a vapor pressure figure can indicate the potential for gas component condensation and suggest the need for a more accurate process simulation. Generally, the potential for partial condensation increases as the operating pressure range of the compressor increases.

To further illustrate this issue and compare alternative scenarios and system configurations, two representative compressor applications are presented. Both applications are based on a common gas composition of 20% methane, 25% ethane, 30% propane, 10% isobutane, and 15% normal butane. Relevant compressor operating parameters for the two examples are outlined in 

Three different operating scenarios are examined for each compressor example. The first scenario is the basic compressor configuration as defined in 

, which is a common compressor layout with the cooled recycle line located downstream of the aftercooler. If any portion of the flow is condensed in the aftercooler, it is assumed that it will flow with the gas phase through the recycle valve and back to the compressor suction. All resulting liquid phase is then assumed to be separated in the compressor suction scrubber before the gas flow is re-introduced into the compressor. A second case is illustrated in 

FIGURE 3: Heat Exchanger Discharge Scrubber (Case 2)

This case represents a configuration where any liquid condensed in the aftercooler will be separated from the recycle flow. Any liquid that is condensed will result in both a reduction in gas molecular weight and gas mass flow rate that enters the recycle line. If there is additional condensation due to the throttling of the gas in the recycle valve, it is assumed that it will be separated in the compressor suction, further reducing the gas mass flow rate and molecular weight.

In the third example, there is no cooling of the recycle flow upstream of the recycle valve, but cooling is provided downstream of the valve and upstream of the compressor suction scrubber. In this case, compressor discharge gas is throttled across the recycle valve first, and any remaining heat of compression is removed in the downstream heat exchanger prior to re-introduction into the compressor suction at the assumed suction temperature. A major drawback of this configuration is the need for an additional heat exchanger, shown in 

FIGURE 4: Downstream Cooled Recycle Flow (Case 3)

Selected results from the low-pressure simulations are shown in 

. These are shown for a range of compressor aftercooler discharge temperatures and assumed common with the recycle valve inlet conditions. The calculated results include the vapor flow molecular weight, the recycle flow to inlet flow mass flow rate ratio, and the comparable recycle flow to inlet flow volumetric flow rate. Changes in molecular weight and compressor inlet volumetric flow rate are particularly important because they correspond to impacts in the operating point of a centrifugal compressor.

It is evident from the simulation results that minimal compressor-operational impacts are associated with wide variations in aftercooler discharge temperatures in the low-pressure example. Some impacts are related to the recycle volumetric flow rates that tend to decrease as the aftercooler discharge temperatures drop. The cause of this volumetric flow reduction is due to the throttling effect of the gas flow across the recycle valve, creating lower recycle valve discharge temperatures and the associated increase in gas density.

A more pronounced sensitivity to compressor-operating parameters with aftercooler discharge temperature is demonstrated in the medium-pressure example. All the selected operating parameters are impacted with more substantial reduction in all parameters with decreasing aftercooler temperature. An exception is illustrated in Case 3, where the aftercooler location is changed to downstream of the recycle valve. Reductions in both gas molecular weight and mass flow ratio are more significant for Case 2—the case that includes any condensate removal downstream of the aftercooler. This relative trend is switched in the volumetric flow reduction and is due to a combination of both gas molecular weight and recycle valve downstream temperature impacts on gas density.

These reductions in gas molecular weight and flows will negatively impact compressor operation with the potential to force the machine offline. The compressor operating point will tend to move toward the surge control line and possibly unable to generate the required pressure ratio. While variable operating speed and the addition of some inlet make-up gas may reduce these effects, it may not be possible to avoid these issues. Of course, constant speed compressor operation or an isolated compressor loop will experience the full influence of these changes. Additionally, variations in gas molecular weight during normal operation may produce similar operating difficulties.

 presents a more graphical illustration of these parameter behaviors on the medium-pressure example.

FIGURE 5: Medium-Pressure Example Simulation Selected Results

Aftercooler discharge temperature is obviously a critical control variable in the prevention of partial condensation and its impact on compressor operation which becomes more influential as compressor-operating pressure range increases relative to component vapor pressures. Methods to control this temperature can be employed both during design and operating phases to minimize or eliminate the resulting operating issues. The most direct solution involves controlling the amount of cooling provided either through the cooling media or the process gas itself. Variation of cooling media flow is dependent upon the type of cooler provided. In the case of water or other liquid media cooling in a shell and tube exchanger, control of liquid media flow is achieved by modulation or bypass with a control valve positioned by a recycle gas temperature controller. Alternatively, fin fan air exchangers may be controlled by either dampers, fan speed, or operation control. Process gas temperature control is limited to cooler bypass control.

FIGURE 6: Recycle Gas Temperature Control Concept

One such concept for process gas control is shown in 

where recycle flows upstream and downstream of the aftercooler are mixed to maintain a temperature setpoint downstream of the recycle throttling valve. Although effective in longer term operation, it does have limitations in startup and other transient operations. Temperature control loops typically suffer from relatively slow response times with significant amounts of dead time when initiated. This is particularly the case when the temperature sensing element is located in a stagnant section of piping as is the case downstream of the recycle valve before it is brought into application. In such cases, setting a preliminary stroke position on the temperature mixing valve based upon some initial conditions may be necessary to prevent condensation when the recycle valve is first opened.

While it is most desirable to address these issues during the design phase of a compressor application, in a number of cases this problem is not realized until the machine has been placed in operation.

Summarizing, aftercooler discharge temperature is both a significant indicator of partial condensing potential and a critical variable to control and prevent condensation. A number of actions can be completed to address the issue and avoid the negative operational impact of partial condensation:

Perform process simulations for the expected range of aftercooler temperatures and gas compositions to determine critical temperatures where condensation is initiated and the extent of the impact on compressor operation.

 If in the design phase, select one or more strategies to control aftercooler discharge temperatures that could include partial bypass of process gas (such as in 

), partial bypass or other temperature control method for cooling media, or movement of the aftercooler downstream from recycle take-off and the addition of another recycle/upstream cooler.

 If in operation, the solution becomes more of a challenge. Potential actions to avoid the associated problems could include manual throttling of coolant with an isolation valve or installation of a new control valve, addition or operation of dampers with air exchangers, and manual reduced operation of cooling fans (cycling fans or shutting off some fans).

 Field verification to ensure that incorporated modifications result in anticipated avoidance of partial condensation and its impact on compressor operation or surge avoidance are recommended.

Mark R. Sandberg, P.E. is the Principal of Sandberg Turbomachinery Consulting. He has more than 40 years of varied experience involving turbomachinery. Additional information is available at sandbergturbo.com.

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Aftercooler discharge temperature was found to be a significant indicator of partial condensing potential and a critical variable to control and prevent condensation.