Table of Contents
1 Introduction
Compressors are one of the most crucial pieces of equipment on oil and gas rigs. The compressors are used to keep the fluid flow constant at the desired pressure. They account for a significant portion of a plant’s capital cost in many applications while being the most energy users in the process. Oil rigs and gas plants have several compressors operating either alone, in trains, or parallel. Any compressor failure may cost the plant millions of dollars per hour, requiring production to stop. The compressor must be regulated to deliver maximum output while utilizing the least power to maximize plant profit.
The most common compressors used in the petroleum industry are centrifugal and axial; both have a limited operating range. Compressor performance may be affected by various causes. A surge is a technically unstable condition that has the potential to damage and disables the compressor. Since centrifugal pumps are sensitive to changing operating conditions, the compressor may get damaged in a few surge cycles. Hence, surge control is a mandatory safety feature for compressor protection in Oil & Gas industry.
2 What is a surge in the compressor?
A surge is an unstable condition of a compressor induced by inadequate or low incoming flow over a certain level. If the inlet pressure is too low and the discharge pressure is too high, the liquid flows backwards. The backward flow of fluid generates an oscillation in pressure and flow, known as a surge. The reverse and forward flow oscillation in the compressor causes considerable vibration. After a few surges, the compressor gets damaged.
There will be a surge line for every compressor’s characteristics, and if the flow or pressure falls below that limit, the compressor will enter into a surge. The surge line is the speed and flow curve that indicates when the surge could occur. Antisurge mechanisms make every attempt to maintain the flow and pressure above the surge line. The following steps describe how the flow cycle repeats:
- As downstream pressure increases to its maximum, the flow will be insufficient when the compressor cannot be further increased.
- If the pressure continues to rise, the flow will diminish and potentially turn negative. Upstream pressure increases as a result of the negative flow.
- At some point, the upstream pressure increase to a level that results in a restoration of positive flow.
- The flow will again increase to reach the surge line. This oscillation will continue if no anti-surge mechanism is employed.
3 What is an Anti-surge compressor system?
Compressors use an anti-surge control mechanism to avoid surges. The flow speed is monitored and controlled to the setpoint in conventional ways to control the surge. The pressure rise is ignored in such systems since only the flow is adjusted to regulate the surge. The conventional method is an extremely inefficient method of controlling the surge. As a result, a more sophisticated anti-surge control system is employed.
An anti-surge control system measures both the flow and the differential pressure across the compressor. The anti-surge system employes a recycle line that consists of an anti-surge valve and surge controller. Recycle line allows the fluid to recirculate through a dedicated pipeline from compressor downstream to upstream to ensure the forward flow. The system’s primary goal is to maintain the flow and pressure below the surge line. The control scheme is dependent on the logic operations that use control lines to operate the control valve in the event of a surge.
A typical anti-surge system measures the parameters, temperature (TT), pressure (PT), differential pressure transmitter (DPT), flow (FT), and compress speed (ST).
An Anti-surge compressor system
The TT, PT, DPT, FT will measure the output variables, and the logic program part of the signal controller will monitor the signals continuously. The signal controller instructs the anti-surge valve to open in the case of a potential surge condition. The valve must respond within 2 seconds to avoid an additional surge. In addition, the valve must be installed as close as possible to the surge line to achieve maximum efficiency for the compressor. The controller can instruct the valve to open fully or partially depending on the process disturbance. The operation of the valve is critical and is frequently overlooked when the valve is not required to be fully open.
Not only does the valve need to open at the correct time, but it must also have the capacity to carry the required flow. And the valve must possess a noise attenuating trim. Because of the high differential pressure across the compressor (1000 psi or greater), noise generation is changed as flow is bypassed around the compressor.
Standard PID controllers were used to controlling surges in early anti-surge systems. However, if the disturbance is sudden, the PID may not be sufficient. Additional fast response controllers can be used to ensure that the valve responds quickly. A selector selects the signals from the regular PID controller and the rapid response controller. To achieve seamless transitions between the two controllers, time-dependent functions are utilized.
4 Why Anti-surge system is required?
An Anti-surge system has the potential to save the manufacturing process from unexpected stoppages. When the flow runs backwards through the rotor wheel, it causes heavy vibration and sound inside the pump. The vibration will eventually harm the compressor’s other components.
The surge primarily impacts the operation of the centrifugal pump. When the fluid flows backwards, it forces the rotor back. The push may damage the rotor bearing, causing the rotor to misalign. Therefore, each surge slightly deforms the thrust bearing. That causes the rotor to get closer to the stationary parts of the compressor case. When a rotor collides with the frame, a piece of the wheel will break and penetrate the compressor’s casing. That means the compressor gets damaged beyond repair, and only a replacement will be able to solve the problem.
Even after replacing the compressor, the surge could occur again and will damage the compressor. An anti-surge compressor system is an efficient solution to resolve this problem.
5 What is an Anti-surge valve?
An anti-surge control valve is the controlling element of an anti-surge control system. The efficiency of the anti-surge system depends on the working efficiency of the valve. It manipulates the flow and pressure in the discharge line. The anti-surge valve ensures an adequate minimum flow of fluid through the compressor, thus reduce the possibility of a surge.
The anti-surge valve is a check valve that opens only when the compressor enters towards surge conditions. When the pressure at the discharge line increases, the valve at the recycle line opens and discharges the pressure to the inlet through recycle line. The resulting increase in the inlet pressure eliminates the surge as the surge happens very quickly so the anti-surge valve must have a fast response.
The anti-surge operation strategy is chosen based on the working of the valve. If the surge variation causes rapidly, then the surge valve setpoints are raised. Another main parameter to consider is the capacity of the surge valve. Considering the volume of fluid that the valve can supply to the compressor inlet and the valve’s reaction time. A predictive controller might forecast surge occurrences and respond to real-world events more quickly while avoiding unnecessary recirculation.
6 Anti-surge valve designing
The anti-surge valve design depends on the capacity and other characteristics of the compressor. The valve, together with its components such as the actuator, positioner, and other accessories, must be fast enough to accomplish the anti-surge process. It’s necessary to have a reliable control valve that works consistently to protect the compressor even under severe conditions. Some of the main design aspects of the anti-surge control valve are:
6.1 Effective sizing
Valves used for most process control applications are sized for maximum flow. But in the case of an anti-surge valve, the valve can not be too oversized or undersized. Anti-surge valve sizing must consider the conditions that may favour surges at various flow requirements and compressor speeds.
An oversized flow for the compressor inlet will result in a chocked flow and, eventually, compressor instability. An undersized flow cannot provide the minimum required volume for fluid to stop the surge. The valve must be suitably oversized to prevent surging.
A reference line is drawn on the compressor map given by the compressor manufacturer to understand the correct valve sizing. The valve size line at which the valve opens (in the case of a predicted potential surge) should always be on the right side of the surge limit. The valve should be sized such that the operating point is on the right side of the surge line at all compressor speeds. The valve can be oversized to a maximum of 10 to 20% above the surge line.
6.2 Rangeability
The minimum and maximum flow capacity in gallons per minute possible through the valve is defined as rangeability. The valve sizing line drawn on the compressor graph determines the required range, the minimum and maximum flow and pressure required above the surge line. High-performance control valves offer rangeability varying from 45:1 to 100:1.
6.3 Speed of response
As mentioned in the earlier sections, the valve must be fast enough to act as soon as a surge occurs. Using the predictive mechanism, the controller will detect the possibility of surge occurrence or process change and instruct the valve to open. Because the flow reversal should happen in a fraction of a second, the valve must react quickly. There is a minimum time required to open the valve gate from the closed position. And the time required is known as stroking time, and it must be minimum. The speed of this operation depends on the type of actuator used, the volume booster, and the length of the valve stroke.
6.4 Tight shutoff
The valve must prevent leakage while it is closed. It negatively affects the efficiency of the anti-surge system. Different types of sealing methods can be considered depending on the range of the application.
6.5 Noise and vibration
Valve manufacturers are required to comply with noise limitations set by plant designers and health and safety authorities. There will be a large pressure difference always across the compressor. This pressure difference is the major cause of vibration in the adjacent piping system. Some types of vibration cause serious mechanical issues with the recycling valve and piping system, while others cause loud noise due to aerodynamics.
Vibration in a pipe system is induced primarily by two factors. First, the sudden change in downstream pressure cannot be detected instantly on both sides of the plug. The valve plug will move axially repeatedly due to the uneven pressure on both sides. Second, vibration is caused by the radial motion of the valve plug while the valve is lifted off the seating surface for the first time. The same pressure differential causes radial motion. Both the axial and radial movements cause excessive vibration in the piping system.
Both vibration concerns may be avoided with proper valve design. To eliminate the axial movement of a plug, a spoked valve plug is used. The pressure differential across the plug can be equalized by using a spoke valve. Radial vibration is reduced by inserting a metal piston ring into a machined groove at the lower end of the valve plug. This ring can prevent leakage that occurs between the valve plug and the cage, as well as any related vibration.
7 Choose the right actuator for anti-surge valve
A fast-changing process necessitates the careful selection of actuators and other peripheral devices. It is critical to select actuators that are both quick-acting and low noise. These are several aspects to take into account while selecting an actuator:
- Because of its fast-stroking speed, electronic actuators were commonly employed for anti-surge valves in the past. However, it is very expensive, with added maintenance costs.
- Double-acting pneumatic actuators are now widely utilized in anti-surge applications.
- The double-acting actuators must be employed with a fail-safe system.
- A bigger control valve necessitates the use of air brake systems. In higher capacity valves, larger actuators will hammer the piston against the top of the actuator. This produces vibration and damages the case.
- For actuators, chrome plating is recommended. It can reduce the friction between the piston and the casing.
- Additionally, a valve booster can be employed with the valve to achieve a faster-stroking speed. The booster increases the amount of air, allowing the valve to open or close quickly.
8 Summary
Designing and maintaining an efficient anti-surge system is the most critical operation in Oil and Gas rigs. A perfect antisurge system not only guarantees the plant’s safety but also enhances profit by reducing frequent plant stoppages due to compressor damage and maintenance.
