The Importance of Accurate Anti-Surge Flow Orifice Sizing for Centrifugal Compressors

As an instrumentation and control (I&C) professional, I have always found that the intricacies of centrifugal compressors are fascinating. Recently, I had the opportunity to work on a project that required me to size an anti-surge flow element assembly. This project made me revisit and reflect on the crucial concept of anti-surge control in compressors, which led me to think that it might be useful to share my insights with fellow professionals in the field.

For anyone who is unfamiliar with this concept or who might need a refresher, let me break it down in simpler terms and explain why accurate sizing of anti-surge flow orifices is essential for the safe and efficient operation of centrifugal compressors.

What is Surge?

Surge is a phenomenon that can occur in centrifugal compressors, and it is something every compressor operator or engineer must be wary of. Surge happens when the compressor operates in a region where the flow through it is insufficient to meet the process demand. In simple terms, surge is a dangerous condition where the compressor begins to behave erratically due to flow instabilities.

Imagine this: You have a centrifugal compressor working to move gas or air from one side of a system to the other. When there’s a sudden drop in flow demand or the downstream equipment is not able to consume the gas, the pressure on the discharge side of the compressor starts to build up. The compressor, however, continues to run. To prevent catastrophic failure, the pressure needs to be released, which is where the anti-surge control system comes into play.

The control system essentially directs the compressor’s discharge flow back to the compressor's suction side through a recycle or anti-surge valve. This process is crucial because, without it, the compressor would continue to cycle into surge conditions, leading to potential damage and failure. These surge cycles occur quickly – within 0.3 to 3 seconds – and can cause vibrations, temperature fluctuations, and generate noise, all of which point to an unstable compressor system.

What Happens in a Surge Cycle?

To better understand surge, let's explore what happens during a surge cycle. In a typical surge event, the following sequence of events unfolds:

• Flow Reversal: As the flow rate through the compressor drops below the surge point, the pressure on the discharge side of the compressor becomes higher than that on the suction side. This causes a momentary reversal of flow.

• Pressure Collapse: This reversal causes the discharge pressure to suddenly drop, restoring forward flow, albeit momentarily.
• Rebuild of Pressure: The compressor then resumes forward flow, trying to rebuild the pressure it lost during the collapse.
• Cycle Repeats: If the root cause of the flow instability (such as low demand) isn’t addressed, this process can repeat cyclically, with the compressor continuously cycling between flow reversal, pressure collapse, and pressure rebuilding.

This cycle can lead to significant instability, causing mechanical stress and vibrations, which can damage components like bearings, seals, and impellers. Additionally, it generates excessive noise and can lead to overheating due to constant fluctuations in flow and pressure.

The consequences of surge cycles are far from trivial. If surge is not prevented or mitigated, the compressor can experience a significantly reduced lifespan, efficiency losses, and even shutdowns, not to mention the safety risks that could arise from prolonged surge events.

Prevention: The Role of Anti-Surge Control

To prevent surge from wreaking havoc on the compressor, anti-surge control systems are designed to ensure that the compressor operates within its safe operating limits. These systems use sensors, control logic, and valves to divert some of the flow back to the suction side, ensuring that the compressor’s flow remains above the surge limit.

Not all compressors need anti-surge control, though. It depends on the type of compressor, operating conditions, and the risk of surge. Here's a quick overview:

1. Compressors That Require Anti-Surge Control: Centrifugal Compressors: Surge is a common issue for centrifugal compressors because their operating range can be quite narrow. Anti-surge control is critical to prevent damage from surge and to maintain stable operation. Axial Compressors: Like centrifugal compressors, axial compressors can also experience surge due to their limited operating range, necessitating anti-surge control for safe operation. A bleed heat valve in a gas turbine axial compressor helps prevent both surge and choke conditions by diverting a portion of the compressed air from the compressor stages. This diversion helps maintain stable operating conditions by regulating the flow and pressure. In addition to controlling flow, the valve helps manage the compressor discharge pressure and temperature, preventing overheating and ensuring optimal compressor performance. By releasing air at specific points, the bleed heat valve stabilizes the flow, preventing surge (which occurs at low flow rates) and choke conditions (which occur at high flow rates). The term "bleed heat" emphasizes the valve's dual role in controlling both flow and compressor discharge conditions to protect the compressor from damage and maintain safe and efficient operation.
2. Compressors That Don’t Require Anti-Surge Control: Positive Displacement Compressors: These compressors (like reciprocating or rotary compressors) operate by displacing a fixed volume of gas or air. They are generally not prone to surge and typically don’t require anti-surge systems. Compressors with Wide Operating Ranges: Some compressors are designed with a broad operating range and are inherently more stable, meaning they are less likely to experience surge.

The need for anti-surge control also depends on process conditions. For instance, in applications where there are frequent fluctuations in flow or pressure, anti-surge control becomes even more critical.

What Is the Anti-Surge Controller?

The anti-surge controller is the brain of the anti-surge control system. It works by continuously monitoring key parameters such as suction pressure, suction temperature, suction flow, discharge pressure, and discharge temperature. This data helps the controller determine how close the compressor is to reaching the surge limit.

By correlating the compressor’s head (pressure) versus flow curve, we can see that minimum flow corresponds to higher head, and maximum flow corresponds to lower head pressure. The head represents the energy imparted to the gas by the compressor to increase its pressure and flow. This is where the anti-surge control system steps in, ensuring that the compressor operates within the safe range – not too close to the surge limit and not too far into the choked flow region.

The anti-surge valve itself is designed to remain closed during normal operations. However, when a process upset occurs, this valve opens, allowing gas to be diverted back to the compressor’s suction side to maintain the minimum required flow.

Sizing the Anti-Surge Flow Element (Orifice)

One of the most critical aspects of anti-surge control is the sizing of the anti-surge flow element. This element must be sized accurately to ensure that it measures the flow rate correctly across a range of operating conditions. If it is not sized properly, the compressor may enter surge or choke conditions, causing process instability.

When sizing the anti-surge flow orifice, we need to consider three main conditions:

1. Maximum Operating Condition: This is the highest flow rate the compressor can handle, usually at maximum speed. The orifice must be sized to accommodate this maximum flow without causing excessive pressure drop, which could reduce efficiency.
2. Normal Operating Condition: This represents the typical operating range for the compressor. The orifice must provide accurate flow measurement under steady-state conditions to ensure reliable anti-surge control during normal operations.
3. Minimum Differential Pressure (ΔP) Condition: This occurs at the lowest flow rate, often near the surge point. The orifice needs to ensure that even at low flow rates, the pressure differential across it is large enough to produce an accurate flow signal. This ensures that the anti-surge control system can detect surge conditions early and take corrective actions.

Types of Orifice Plates and Their Usage

• Concentric Orifice Plate (Square Edge): Design: The most popular concentric orifice plate typically features a sharp, square edge central hole. Usage: Most common type, suitable for clean, single-phase fluids (liquids, gases, or steam) which is critical for accurate flow measurement. Advantages: Simple, cost-effective, and accurate for standard flow measurement. This design helps in creating a distinct pressure drop and allows for more precise differential pressure measurements. Square-edge orifice plates are commonly used in applications where flow measurement accuracy is important, especially in low to medium flow conditions.

• Concentric Orifice Plate (Beveled Edge): Design: Central hole with a beveled downstream edge. Usage: Suitable for high-velocity or high-pressure fluids, reducing turbulence and pressure recovery issues. Advantages: Improves flow stability and reduces cavitation risk.

• Quadrant Edge Orifice Plate (Rounded Edge): Design: Central hole with a rounded inlet edge. Usage: Ideal for low Reynolds number flows, such as viscous fluids or low flow rates. Advantages: Improves accuracy and reduces pressure losses for high-viscosity fluids.

• Eccentric Orifice Plate: Design: Hole located off-center. Usage: Used for fluids with suspended solids or slurries to prevent clogging. Advantages: Allows solids to pass freely, reducing obstruction and maintenance.

• Segmental Orifice Plate: Design: Partial circular opening (segment-shaped). Usage: Suitable for dirty or viscous fluids, slurries, or two-phase flows. Advantages: Prevents clogging by allowing solids or heavy phases to bypass.

• Orifice Plate with Vent or Drain Hole: Design: Concentric plate with an additional small vent hole (top) or drain hole (bottom). Usage: Used in systems where trapped gas (in liquid flow) or liquid (in gas flow) needs to be removed. Advantages: Ensures accurate measurement by preventing phase accumulation.

• Restriction Orifice Plate: Design: Small hole specifically sized for controlled pressure drop. Usage: Used for reducing pressure, limiting flow, or noise suppression in pipelines. Advantages: Precise pressure reduction without introducing turbulence.
• Chamfered Edge Orifice Plate: Design: Edges are chamfered (tapered) for durability. Usage: Useful in abrasive or high-velocity fluid systems where wear resistance is required. Advantages: Reduces edge deterioration and maintains accuracy over time.
• Knife Edge Orifice Plate: Design: Extremely thin, sharp-edged hole like a blade. Usage: Rarely used, but suitable for highly precise applications. Advantages: Exceptional precision, though less durable and prone to wear.

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Comprehensive Guide to Anti-Surge Flow Measurement: Key Parameters and Best Practices

Flow measurement for anti-surge control in centrifugal compressors is a critical aspect of ensuring safe and efficient operations. A precise flow measurement system prevents surge conditions, ensures compressor stability, and protects valuable assets. While the concept may seem straightforward, its execution requires meticulous planning, accurate data, and strict adherence to international standards like ISO 5167.

To achieve this, let’s delve into the essential parameters and specifications you must consider when designing, procuring, and installing a flow element and its associated instrumentation.

Key Parameters for Accurate Flow Measurement

When sizing the anti-surge orifice plate or selecting another flow measurement device, the following parameters must be provided to the manufacturer:

1. Compressor Data Compressor Speed: Rotational speed, often tied to flow characteristics. Volumetric Flow: The actual flow rate under operating conditions. Mass Flow: The total mass of gas moving through the system, independent of pressure and temperature. Normal Flow: Standardized flow rates based on reference conditions.
2. Suction Side Parameters Operating Pressure and Temperature: Ensure accurate flow corrections. Design Pressure and Temperature: For safety and robustness of the orifice and associated piping.
3. Gas Properties Density: A key parameter for mass flow calculations. Compressibility Factor (Z): Adjusts for deviations from ideal gas behavior. Specific Heat Ratio (k): Essential for pressure-drop calculations and assessing flow stability. Molecular Weight: Defines the gas's mass properties. Viscosity: Impacts flow dynamics and pressure drop across the orifice.
4. Orifice Design Parameters Minimum and Maximum DP Ratios: To cover the entire range of expected flow conditions. DP × ksquared: Corrected differential pressure values for varying gas characteristics. Beta Ratio: The ratio of orifice diameter to pipe diameter under all three conditions: Maximum Sizing Point: The upper operational limit. Normal Point: Typical operating range for stable flow. Minimum DP Point: Near the surge limit, where precision is critical.
5. Pipeline Data Pipeline Size: Diameter of the pipe housing the orifice plate. Pipeline Thickness: Affects the overall system rigidity and flow characteristics. Disc Expansion Factor (mm/°C): Thermal expansion rate of the orifice plate material to ensure proper fit and accuracy over temperature variations. Piping Expansion Factor (mm/°C): Thermal expansion rate of the pipeline material to account for differential expansion between the pipe and orifice plate. Acceptable Permanent Pressure Losses: The maximum allowable pressure drop through the orifice, typically expressed as a percentage of the system pressure.

Procurement Specifications

When preparing purchase specifications for the orifice plate and flowmeter, ensure the following points are addressed:

1. Sizing the Orifice Request the manufacturer to perform orifice sizing calculations using the data provided, ensuring that upstream and downstream pipeline lengths meet ISO 5167 requirements. Verify the calculated pressure drop values across the orifice, ensuring they fall within acceptable limits. Check the permanent pressure loss and confirm it does not exceed the system's operational tolerance.

2. Installation and Material Details Specify the mounting connection type (e.g., flanged or threaded) and mounting material to suit process conditions and corrosion resistance requirements.

3. Instrumentation Selection Choose a differential pressure transmitter that can handle the full range of expected DP values. Ensure compatibility with the process gas and its properties for wet part/sensing material of the transmitter. For gas compositions containing H? or H?S, 316L stainless steel can be used for wet parts if the H?S concentration and operating conditions are within the limits specified by NACE MR0175/ISO 15156 for sour service. However, for higher concentrations of H? or more severe environments, materials like Hastelloy or Inconel are preferred due to their superior resistance to hydrogen embrittlement and corrosion. Another key selection criteria in Oil & Gas industry is to consider the hazardous area classification. For a Zone 2 IIB T3 hazardous area, select a pressure transmitter certified for Ex-eC or Ex-nA or Ex-ic protection methods, ensuring it meets the required gas group (IIB) and temperature class (T3, ≤200°C), and specify the required certification (e.g., ATEX, IECEx, CSA, or FM) based on the country-specific installation standards. Ensure the process connection, electrical connection type and size, mechanical protection class, display unit, output signal (e.g. 4-20mA with SMART HART) and housing material are specified based on the site’s environmental conditions to facilitate appropriate supplier selection. For applications with wide flow ranges, consider using two DP transmitters: Low-Range Transmitter: For accurate measurements near minimum flow conditions. High-Range Transmitter: For measuring up to the maximum DP.

4. ISO 5167 Compliance The orifice and associated flowmeter must comply fully with ISO 5167 standards for flow measurement. This ensures standardization, reliability, and compatibility with other components in the system. According to ISO 5167, orifice plates for flow measurement must meet the following requirements: precise bore accuracy and sharpness for consistent flow separation, concentricity to avoid misalignment errors, smooth surface roughness to reduce turbulence, and flatness to prevent distortion. The plate material should suit the process fluid, typically stainless steel for general use. A beta ratio between 0.1 and 0.75 ensures accuracy: a low ratio (0.1) improves measurement accuracy at low flows, while a high ratio (0.75) is better for higher flow rates. Straight upstream and downstream pipeline lengths are required for stable flow profiles. The design must support process indication, control, safety trip accuracy, and reliable totalization, ensuring accurate cumulative flow measurement over time.

Ensuring Flow Accuracy Across All Conditions

The flowmeter and associated system must be capable of measuring gas flow across all operating conditions outlined in the compressor datasheet and its expected performance curves. This includes handling fluctuations in suction pressure, temperature, and flow rate without significant loss of accuracy.

For improved accuracy:

• Two DP transmitters can be configured with overlapping ranges to ensure precise readings at both low and high flow conditions.
• The system should dynamically adjust flow measurements for variations in pressure, temperature, and gas properties using real-time corrections based on gas equations like the ideal gas law and compressibility factors.

Collaborative Validation and Final Checks

Before finalizing the orifice and flow measurement setup:

1. Work closely with the manufacturer to ensure their sizing calculations align with your specifications.
2. Verify the calculated pressure drop and permanent loss values, confirming they meet your system’s operational and efficiency requirements.
3. Conduct a review of the entire system, ensuring all components (orifice, piping, and instrumentation) are compatible with process conditions and meet required safety standards.

By addressing these parameters and ensuring a robust procurement process, you can achieve precise, reliable, and safe flow measurement for anti-surge control in centrifugal compressors. This meticulous approach safeguards equipment, enhances performance, and minimizes risks of surge or inefficiencies in the system.

Why Are Pressure and Temperature Corrections Necessary?

In real-world conditions, the gas flowing through the compressor will experience variations in pressure and temperature. These variations can significantly impact the density and flow rate of the gas, making it necessary to apply pressure and temperature corrections to ensure accurate flow measurement.

The ideal gas law plays a key role here. When the gas expands or contracts due to temperature or pressure changes, its volume and density will also change. If we didn’t apply pressure and temperature corrections, we might misinterpret the actual flow rate, leading to incorrect anti-surge control actions. For example, a lower temperature or a higher pressure increases the density of the gas, resulting in a lower volumetric flow rate for the same mass flow rate.

Therefore, by applying the appropriate corrections to account for pressure and temperature variations, we ensure that the flow measurements are standardized to reference conditions, providing the anti-surge control system with accurate data for decision-making.

Settle Out Pressure and Choke Region

Now, let’s delve into the concepts of Settle Out Pressure and Choke Region, both of which are closely linked to the operation of the anti-surge system.

• Settle Out Pressure: This is the pressure that the compressor will eventually stabilize at once the anti-surge valve begins to divert flow back to the suction side. The flow conditions may not stabilize immediately, so the system needs time to “settle out” to the desired operating pressure. If the valve is not properly sized or operated, the compressor could experience unstable pressure conditions, risking damage.
• Choke Region: The choke region refers to the point where the compressor is operating at its maximum flow capacity for a given pressure ratio. A choke condition in a centrifugal compressor occurs when the flow rate through the compressor exceeds its design capacity, causing the flow to become unstable. This typically happens when the compressor operates at very high flow rates, leading to a drop in pressure and efficiency. In this region, the compressor cannot handle any more flow, and any additional flow will not increase the pressure ratio. If the compressor operates in the choke region for extended periods, it can lead to inefficient performance and potential damage to components like the impeller and bearings.

In a choke condition, the compressor's ability to increase flow is limited, and it may result in significant damage or performance issues if not controlled, often requiring anti-surge systems to protect the compressor. The anti-surge valve is critical in managing these regions, especially when transitioning from normal operating conditions to the choke region. The valve helps modulate the flow to prevent the compressor from reaching choke conditions or from entering surge, both of which can be damaging.

The Role of the Anti-Surge Valve briefly

The anti-surge valve plays a pivotal role in maintaining a stable compressor operation. It does so by controlling the flow from the discharge back to the suction side, ensuring the compressor remains in the safe operating range. In the event of surge, the valve diverts gas flow, allowing the compressor to stabilize and return to normal operating conditions. The valve’s proper sizing and operation are crucial in keeping the compressor out of both surge and choke conditions.

Conclusion

The anti-surge flow orifice is a small but critical component in centrifugal compressor systems. Its accurate sizing, considering maximum operating conditions, normal operating range, and minimum differential pressure, ensures precise flow measurement across the entire performance envelope. By integrating all these conditions during the design phase, we can achieve reliable anti-surge control, safeguard compressor health, and optimize operational efficiency.

This meticulous approach highlights the engineering discipline required in compressor systems, underscoring the value of detailed design and operational analysis in achieving robust and reliable performance.