October 2024

Special Focus: Valves, Pumps, Compression and Turbomachinery

Demystify warm-up practices for centrifugal pumps

This article provides a thorough analysis of various warm-up practices—examining their benefits and drawbacks and proposing well-grounded guidelines for the development of efficient warm-up systems.

IP: 18.223.158.132

The design and operational efficiency of warm-up mechanisms in hot centrifugal pumps play a pivotal role in preventing their premature failure in high-temperature applications. It is crucial to achieve and maintain a uniform temperature within these pumps to ensure the integrity of internal clearances, enhance performance and extend the equipment’s lifespan.  

Consulting the manufacturer’s recommendations is paramount for identifying the most effective warm-up procedures. Uniform warming of the pump is essential; uneven warm-up flow or an insufficient flow volume can lead to casing distortion, rotor bowing or both. This article provides a thorough analysis of various warm-up practices—examining their benefits and drawbacks and proposing well-grounded guidelines for the development of efficient warm-up systems. While pump vendors typically advise on appropriate warm-up techniques, their involvement is often limited during the early stages of process and instrumentation diagram development. This article aims to bridge this gap by offering practical guidance, thereby minimizing potential late-stage modifications in projects. 

The necessity of warm-up lines. Per API RP 686,1 it is essential for pump processing materials above 150°C (300°F), or those with a high pour point, to include warm-up lines to achieve and maintain the necessary temperature of the pump. In process plants, when the temperature difference between the ambient environment and the service liquid exceeds 100°C (212°F), installing a warm-up system becomes a consideration to prevent thermal shock in pumps. 

The necessity of this system is underscored in scenarios involving parallel centrifugal pumps, where one pump operates while the other serves as a standby. The operational pump may run for extended periods, leaving the spare pump idle. This situation, coupled with a significant difference between the ambient and liquid temperatures, can cause the spare pump’s components, such as the impeller and casing, to reach ambient temperature. Should the operational pump fail, the introduction of hot liquid into the previously idle, cold pump could induce thermal shock, risking cracks in the pump casing and damage to other components. 

Pre-warming the pump and ensuring a continuous warm-up flow to an idle pump are critical to maintain dimensional thermal uniformity. This uniformity is essential to preserve internal clearances, pump efficiency and longevity. Distortion from thermal shock, such as shaft bowing, can lead to internal rubbing, pump seizure, mechanical seal damage, high vibration, and wear on bearings and their clearances. 

Thermal stratification within pumps can occur with rapid cooling or improper warming, leading to a thermo-siphon effect with hot liquid at the top and cooler liquid at the bottom of the pump casing (FIG. 1). This is most common in between bearing pumps where the inlet and outlet nozzles are both at the top of the pump case. This condition causes unequal thermal expansion of the casing and rotor, potentially resulting in galling between components, vibration and seizure. Instances have been observed where spare hot pumps developed leaks due to a lack of warm-up provisions to the casing, highlighting the need for comprehensive warm-up practices beyond just the discharge area of the pump. 

FIG. 1. Thermal stratification within a pump. 

Different warm-up system arrangements. The methodology for warming up hot-service pumps varies: each approach presents its own set of advantages and disadvantages. Here, the authors will evaluate the different types of warm-up systems, excluding less-favored methods, such as drilling a hole (approximately 3 mm–5 mm in diameter) in the discharge check valve.  

Draining through the pump casing drain. This method allows the pumped liquid, under positive pressure at the suction, to drain out through the standby pump’s casing drain connection to a lower pressure point, such as the slop system (FIG. 2).

FIG. 2. Draining through the pump’s casing. 

Despite its potential for use, this practice has not been widely adopted due to the following significant drawbacks: 

• Excessive slop generation: This approach contributes to the production of excessive slop, thereby reducing refinery margins. The process of draining to slop systems can lead to increased handling and processing of waste materials, thus impacting the overall efficiency and profitability of refining operations. 

• Thermal shock to discharge piping: The discharge piping, extending from the pump discharge to the isolation valve, lacks warm-up flow. This absence of warm-up flow subjects the piping to thermal shock when the standby pump is initiated, potentially compromising the integrity of the piping system and increasing the risk of failures. 

• Drainage limitations: Underground piping is designed to withstand temperatures up to a maximum of 60°C (140°F); therefore, hot fluids cannot be discharged into these pipes. The necessity to drain the warm-up liquid to an aboveground closed drain system, rather than to an underground one, presents logistical and environmental challenges. This requirement can complicate the warm-up process by necessitating additional infrastructure and potentially increasing the environmental impact due to the handling of drained materials. 

External hot fluid to pump suction. A case study shared at an industry conference highlighted an atmospheric residue desulfurization feed pump experiencing repeated seal failures due to inadequate warm-up. The initial warm-up arrangement, as designed by the engineering company, deviated from the vendor-recommended practices (FIG. 3). Modifications to align with these recommendations were not made due to potential startup delays. 

FIG. 3. External fluid to suction FSD. 

The method. To warm up the standby pump, a 2-in. hot diesel line from the fractionator was connected to the pump suction, with the suction valve closed and a 1.5-in. suction bypass open. This setup was intended to establish warm-up liquid flow through the pump casing to the feed surge drum (FSD) via a minimum-circulation line, and through the suction line to the FSD. However, this method failed to adequately warm the pump’s casing, leading to repeated seal failures. Following this, the warm-up arrangement was altered to adhere to the vendor’s recommended practice. 

Effect of inefficient warm-up. The warm-up liquid tended to flow through the path of least resistance, typically the suction line, thus failing to adequately warm the casing.  

Bypass across the discharge check valve. A reverse-flow bypass line is installed around the discharge check valve to facilitate warm-up. API RP 686 Fig-B.3 shows spillback across the check valve, as shown in FIG. 4. For multi-stage pumps or those with large casings, this alone might not suffice. Additional warm-up lines connected to the pump casing drains may be necessary. 

FIG. 4. Bypass across the discharge check valve. 

Drawbacks of API recommended practices. To mitigate the risks of reverse flow, keeping the discharge isolation valve of the standby pump open is necessary to establish warm-up flow. If equipped with a single non-return valve (NRV), there is a high risk of reverse flow, potentially causing reverse rotation of the pump. Cracking the isolation valve open slightly reduces this risk but can damage the valve’s seat, complicating pump isolation. 

An alternative is to route the bypass across the discharge isolation valve and check valve, necessitating a spade in the bypass line for positive pump isolation during maintenance. Additionally, if two discharge isolation valves are separated by a spool piece, a branch from the warm-up line should route to this spool piece, equalizing pressure across the isolation valve during pump startup. 

Additional observations include direct connection drawbacks. Connecting the warm-up line directly to the pump’s bottom without linking it to the discharge header keeps the header cold for the standby pump. Starting this pump can rapidly warm the discharge header flanges, risking leaks. 

The selection and implementation of a warm-up system for hot-service centrifugal pumps must be guided by a thorough understanding of each method’s advantages and limitations, as well as adherence to vendor recommendations and a commitment to optimizing operational safety and efficiency. The choice of warm-up system is influenced by various criteria: 

1. The number of pump stages (whether a single stage or multiple stages) 
2. The size of the pump, which is determined by the required brake horsepower
3. The configuration of suction and discharge entries (whether they are on the side or the top) 
4. The placement of bearings (whether they are overhung or located between bearings) 
5. The design of the casing (whether it is circular or barrel-shaped). 

This evaluation serves as a foundational guide for engineers and designers in making informed decisions that align with best practices and industry standards, ultimately enhancing the reliability and lifespan of critical pump machinery. 

Flowrates through warm-up lines: Key criteria and practices. The flowrate through the warm-up line is a critical factor in ensuring the effective and safe operation of warm-up systems for pumps. To optimize this flowrate, several criteria must be met: 

• Warm-up rate: The flow should facilitate a warm-up rate of approximately 20°C/hr–30°C/hr. This rate is essential to gradually bring the pump up to the required operational temperature without causing thermal stress or damage. This rate should be confirmed with the pump vendor during detailed design. 

• Temperature maintenance: The flow must be sufficient to compensate for heat loss to the ambient environment, thereby maintaining the desired temperature within the pump [within 28°C–35°C (82°F–95°F) of the process operating temperature]. This balance ensures that the pump remains at a steady operational readiness state.  

• Avoidance of reverse rotation: The flowrate must be carefully controlled to prevent it from being so high as to risk causing reverse rotation of the idle pump. Reverse rotation can lead to mechanical damage and operational issues. 

In terms of sizing, a good engineering practice is to design the bypass line to handle 2% of the pump’s normal flowrate or to ensure a minimum size of a 0.75-in. line. This approach provides a baseline to ensure that the flowrate through the warm-up line is adequate, yet safe. 

Additionally, the selection of valves for the bypass line requires careful consideration. Globe valves or angle valves are typically recommended due to their ability to manage flowrates effectively and to accommodate high pressure drops. For positive isolation of the pump, an additional gate valve with a spade blind is recommended for the warm-up line. The potential for high pressure drops across the valve necessitates extra attention to the valve’s design and selection to ensure that it can withstand the operational conditions without compromising the system’s functionality and safety. 

           Vendor confirmation. It is important to confirm the design warm-up flowrate with the pump vendor. Pump vendors possess the detailed specifications and operational understanding necessary to advise on the optimal flowrate that meets all the required criteria without exceeding the pump’s design limitations.  

Best practices for incorporating ROs in warm-up lines. Installing a restriction orifice (RO) in the warm-up bypass line is a critical consideration in the design of high-differential-pressure pumps used in hydroprocessing units. This measure aims to prevent reverse rotations of pumps when throttle valves are fully open. However, introducing ROs into the system is not without challenges, particularly due to the risk of fouling, which can compromise flow and efficiency. The conventional assumption is that a standard 0.75-in. line would limit the flow to approximately 2% of the pump’s normal flowrate. Yet, in practical scenarios—such as a hydroprocessing unit with a feed pump rated at 400 m3/hr, operating at a discharge pressure of 110 barg and a suction pressure of 5 barg—the actual flow through a 0.75-in. line, especially for diesel at 150°C (302°F), can significantly exceed this percentage, reaching as high as 6% (25 m³/hr) of rated flow.  

Given the high-pressure nature of these pumps and the strategic placement of pump discharge emergency shutdown (ESD) valves at a considerable distance from the pump, it becomes imperative to opt for a minimum warm-up line size of 2 in. This choice of a 2-in. line ensures the mechanical integrity of the piping over long distances. 

In hydroprocessing units, where low-pressure (LP) sections (like the feed section) interface with high-pressure (HP) sections (such as the reactor circuit), the integrity of the LP section is paramount. It must be protected against overpressure scenarios potentially triggered by gas blowby if the operational pump fails. High-integrity ESD valves are employed to mitigate the risk of reverse flow, thus serving as a primary defense for the LP section. Given the considerable demands on relief systems, and the impracticality of using relief valves due to their size and impact on flare design, the warm-up line plays a pivotal role in managing relief loads, effectively bypassing the pump’s discharge ESD valve. 

Specifically, the RO is designed with a minimum diameter of 6 mm to ensure it remains free from blockages and maintains consistent and reliable flow throughout the warm-up system. This specification is crucial to maintain safety in scenarios where HP and LP sections interact, thereby making the RO a safety-critical component. To mitigate the potential issues of noise and vibration, which can arise from the high-velocity flow through the RO, a multi-stage restriction orifice design is recommended. This approach allows for gradual pressure reduction across stages, minimizing the risk of noise, vibration and wear on the system. 

Although the inclusion of an RO in warm-up lines introduces complexity, its role in safeguarding the system’s operational integrity and safety is undeniable. By carefully considering the size and design of the RO, along with the overall configuration of the warm-up system, engineers can ensure effective protection against overpressure scenarios while maintaining flow control and mechanical integrity. 

Minimum instrumentation requirements for pump warm-up phases. For effective monitoring and control during the pump warm-up process, it is crucial to ensure that temperature variations are within safe limits. To achieve this, adequate instrumentation is essential to measure and confirm temperature differences across key points of the system. Specifically, the temperature difference across the pump should not exceed 28°C–35°C (82°F–95°F) when compared to the normal operating temperature. Adhering to this guideline helps in preventing thermal stress and ensuring the integrity of the pump during warm-up phases. 

The following are common instrumentation practices:  

• Temperature indicator in the warm-up line: Installing a temperature indicator in the warm-up line allows for real-time monitoring of the fluid temperature as it enters the pump. This measurement is vital for adjusting the warm-up flow to maintain the desired temperature gradient. 

• Suction header temperature indicator: A temperature indicator on the suction header ensures that the temperature of the fluid being drawn into the pump is monitored. This helps in assessing the temperature differential between the incoming fluid and the pump body, aiding in the adjustment of the warm-up process.  

• Skin temperature indicators on the pump casing: Typically, these are used for insulated pumps or where measurement of the pump casing with a handheld temperature measuring device is impractical. To directly assess the temperature of the pump casing and ensure uniform heating, at least four skin temperature indicators are strategically placed around the pump casing. These indicators are crucial for detecting any abnormal hotspots or variations in casing temperature, which could indicate issues with the warm-up procedure or pump operation. 

The implementation of these minimum instrumentation requirements is fundamental to the safe and efficient warm-up of pumps in process systems. By closely monitoring temperatures at these critical points, operators can ensure that the pump and the system are adequately prepared for normal operation, minimizing the risk of thermal shock and maximizing the longevity of the equipment.  

         The necessity of flushing oil connections in pump systems. The incorporation of a flushing oil connection in pump systems—particularly those handling hydrocarbons with low pour points—is a crucial design consideration. This feature is specifically aimed at mitigating the risks associated with the congealing of fluids within the pumps and their associated piping when the system is removed from service. 

         Challenges without a flushing oil connection. Operators often face significant operational challenges with pumps that do not have a flushing oil connection, especially when dealing with hot diesel pumps. One common issue arises when attempting to drain these pumps after isolation. Since underground drainage systems are typically designed to accommodate fluids up to a maximum of 60°C (140°F), operators are forced to endure lengthy waiting periods for the pump inventory to cool down to acceptable levels. Despite these delays, there remains a risk of encountering pockets of hot diesel trapped within the insulated pump and piping system. 

Moreover, the absence of a flushing oil connection complicates the process of priming the pump for restart after maintenance. Operators may resort to opening the suction valve to prime the pump, inadvertently subjecting the cold pump to a rate of heating that far exceeds the recommended maximum of 20°C/hr–30°C/hr. This not only poses a risk to the pump’s integrity, but also to the safety of the operation.  

          Advantages of implementing a flushing oil connection. A hard-piped flush connection, ideally positioned in the discharge line, addresses these challenges by enabling the safe and efficient priming and cooling of the pump. The benefits of this practice include: 

• Enhanced safety: By facilitating the controlled cooling of the pump, the risk of accidents related to thermal shock, or the handling of overheated equipment, is reduced. 

• Improved operability: The flushing oil connection allows for a more rapid and efficient preparation of the pump for maintenance or restart, reducing the downtime associated with waiting for the pump inventory to cool. 

• Operational efficiency: The ability to prime the pump by using a designated flushing oil connection streamlines the startup process, ensuring that the pump can be brought back online quickly and safely following maintenance activities. 

The implementation of a flushing oil connection in pump systems is a good practice that significantly contributes to the safety, operability and efficiency of handling hot hydrocarbon fluids. By enabling the safe priming and cooling of pumps, this design feature effectively reduces operational delays and enhances the overall reliability of the pumping system. 

Best practices for warm-up line arrangements during front-end engineering design (FEED). In the FEED stage, optimizing the project schedule is a priority for most refiners. With the trend toward adopting a FEED-plus approach [which aims for minimal changes during the engineering, procurement and construction (EPC) phase], it becomes essential to incorporate conservative, yet effective, design practices early on. This approach is particularly crucial for the warm-up line arrangement of hot-service pumps, ensuring operational efficiency and safety from the outset. As a guideline, the following practices are recommended during the FEED stage, subject to confirmation during the detailed design stage from the pump vendor: 

• Warm-up line placement: The warm-up line should be configured to cross the last discharge isolation valve, extending toward the discharge header. This arrangement ensures that both the pump and its associated piping can be adequately warmed up, while also maintaining the pump’s discharge valve in a closed position during warming. 

• Connection points: The warm-up line connection should be dual-purpose—i.e., it must connect both to the discharge header and the bottom of the pump casing. This dual connection facilitates efficient temperature management by allowing for the warming of the pump body and associated piping. 

• Temperature monitoring: A temperature indicator should be installed on the warm-up line and another on the pump suction line. These indicators will provide crucial data on temperature differentials, ensuring that the pump is not subjected to excessive thermal stress during warm-up phases or operation.  

• Casing temperature monitoring: It is important to specify in the pump datasheet the requirement for an adequate number of skin temperature indicators (at least four) on the pump casing for the pumps in critical service. This measure is vital for the proper monitoring of casing temperatures, allowing for early detection of any abnormal thermal patterns that could indicate operational issues. 

• Cold diesel or flushing oil connection: Even for pumps designed for hot diesel service, ensuring the provision for a cold diesel or flushing oil connection is crucial. This feature enables the safe priming and cooling of pumps, and thus effectively reduces operational delays and enhances the overall reliability of the pumping system. 

• Considerations for pumps seals in hot flashing services: An important consideration for pumps in hot flashing services, such as hydroprocessing feed pumps and boiler feedwater pumps, is the management of seals during commissioning. Introducing hot warm-up fluid into empty, unpressurized pumps—while utilizing API Plans 21, 22 or 23, or any other plan involving process fluid through a cooler for seal flushing—can result in flashing across the seals and potentially cause damage to the seal faces before the pump is started.  

One mitigation strategy is to connect an additional colder-fluid line (such as for cold flushing oil or cold condensate) to the pump, allowing colder fluid to be introduced gradually before the hot warm-up fluid. Prior filling of the pump with colder fluid results in slow pressurization of the seal, thus potentially avoiding pressure shock due to the sudden flashing of the hot fluid into an empty pump and its seal system.  

A proposed warm-up arrangement during the FEED phase is shown in FIG. 5. 

FIG. 5. Proposed warm-up arrangement during the FEED phase. 

Adhering to these best practices during the FEED stage for warm-up line arrangements in hot-service pump systems can significantly contribute to a project’s success. By planning for efficient temperature management, monitoring and operational flexibility, refiners can minimize the risk of thermal stress and mechanical failure, ensuring smoother transitions into the EPC phase with minimal design alterations. It is imperative that these guidelines be refined and confirmed with the pump vendor during the detailed design stage to adapt to specific operational requirements and vendor recommendations.  

Takeaways. The authors have observed installations of check valves downstream of minimum-circulation valves, and such installations in these lines, particularly when returning to the suction vessel, present ambiguity regarding their rationality and necessity.  

The authors have also observed the installation of isolation valves at the inlet of minimum-circulation valves [flow control valves (FCVs)] in some designs. The purpose of adding an isolation valve at the inlet of an FCV is to allow drainage of only the upstream and downstream sections of the FCV during maintenance, rather than requiring the drainage of the entire pump-associated system. However, an isolation valve installed upstream of the minimum-circulation valve (FCV) without requiring it to be car seal open or locked open may pose a safety and integrity risk.  

REFERENCES  

1 American Petroleum Institute (API), “API RP 686: Recommended practice for machinery installation and installation design,” 2nd Ed., December 2009. 

2 Heald, C. C. and D. G. Penry, “Design and operation of pumps for hot standby service,” online: https://www.911metallurgist.com/blog/wp-content/uploads/2016/01/Design-and-Operations-of-Pumps-for-Hot-Standby-Service.pdf  

3 Toghraei, M., Piping and Instrumentation Diagram Development, John Wiley & Sons Inc., March 2019. 

4 Karassik, I. J., J. P. Messina, P. Cooper and C. C. Heald, Pump Handbook, 4th Ed., McGraw Hill, December 2007. 

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