Uprating and Rerating InCan Your Equipment Handle Operational Changes? What to Know About Uprating and Reratingdustrial Equipment: A Systematic Approach to Higher Loads and Modified Operations

Figure 1. An example of machinery substructure reconfigured and rerated.

Engineers and operators in chemical processing plants are increasingly required to evaluate equipment and machinery for higher loads, greater stresses, elevated operating pressures, increased capacity and other more demanding operational parameters.

Additionally, they must reassess existing equipment to accommodate new or modified operating conditions. Despite the growing frequency of these rerating and uprating projects, there is a notable scarcity of published reports about the methods. This article outlines a systematic approach engineers can use to assess the reliability, safety and longevity of critical equipment under evolving operational demands.

Uprating for increasing equipment demands is closely related to the maximum operational loads the equipment or machinery can handle before damage, failure or potential malfunction will occur.

This means assessments for new operational cases must account for all possible failures and issues. This is applicable for rotating equipment, fixed equipment, processing packages, bulk material handling systems, such as conveyors, among others.

The uprating and rerating processes should also involve all equipment components to identify whether they contribute to potential weaknesses and bottlenecks.

Operational and Mechanical Considerations

When uprating or rerating, the equipment should be strong enough to withstand higher stresses, dynamic loads, shock, twist, vibration and other adverse effects for the intended new operation. Along with strength, another important consideration is to ensure adequate stiffness, such as bending stiffness, exists. The deflections and deformations should still be small under new operational loads. This, along with good dynamic behaviors, mandate high overall stiffness. Corrosion, erosion and temperature changes may need to be considered and properly assessed.

The mechanical and operational assessment for the uprating and rerating is typically a complicated process with many steps and stages. Usually, many parameters and factors are involved and some of them, such as failure criteria, corrosion rate, fatigue and brittle facture, are often subjected to uncertainties and changes based on operational and environmental conditions. The matters are too complicated and too nonlinear to be calculated or simulated easily and accurately. The overall equipment or machinery should remain fully and reliably functional under new operating cases for the intended life.

Case Study: High-Pressure Piping System Modification

The following involves a large-diameter 12-inch (DN300) piping system with an outside diameter of around 323 millimeters and thickness of around 11.5 millimeters. This system was part of a high-pressure unit in a chemical processing plant.

This portion of piping operated under normal conditions in the existing operating cases. However, when new operating cases were introduced to the unit, the piping became subject to cold gas flow during depressurization, as the high-pressure gas cooled due to the Joule-Thomson effect.

The piping system began with a 2-meter straight section leading to the first valve. From this valve, a 3-meter straight run continued to a junction area containing three tees and connections that linked various facility lines. Beyond this junction, the system included a 90-degree elbow, followed by approximately 2 meters of pipe leading to the second valve. Finally, the line connected to the main header through another 90-degree elbow and downward piping from above.

At normal operation, this piping system was subjected to pressure of 141 Barg at 60°C. With the new operating procedure, the temperature of cold gas can be as low as -43°C. Using two valves, there could be different scenarios where a portion of this piping is under operating pressure whereas another section is subjected to cold temperatures due to the Joule-Thomson effect.

Given the considerable thermal movements expected, all supports were modified to resting supports only during the piping system modification. These supports provide no restriction on lateral or longitudinal movements.

In normal operations, the pressure of 141 Barg resulted in hoop stress above180 Mpa. Such a high hoop stress is the signature characteristic of high-pressure, large-diameter piping and any other stresses — resulting from bending, torsion or axial, for example — can add to this high-hoop stress and move the stress toward the limit.

On this piping during normal operation, the maximum stress is around one of the tee connections, and the stress value is around 60% of the allowable stress. However, the maximum stress, overall, considering new cases, is experienced in another scenario where a portion of piping is under pressure and another portion is under cold conditions (new cases of depressurization).

In such a case, the thermal movement of the low-pressure cold section cause thermal loads and consequently thermal stresses in forms of axial and bending stresses in the pressurized section. These stresses combined with high hoop stress results in the maximum stress more than 71% of the allowable stress in that scenario.

Parametric Studies and Sensitivity Analysis

Parametric studies and sensitivity analyses are important in many uprating and rerating cases because they show the relationship between the new equipment demands and how the equipment will perform under certain variables.

For instance, if uprating for a higher operating pressure is necessary, it’s important to examine how the increase of pressure would affect the stresses and other limiting factors of the equipment through a set of properly planned sensitivity analyses and parametric studies.

Parametric studies involve varying key parameters, such as operating pressures, capacity or operating temperatures, to see how limiting factors like induced stresses would change. Sensitivity analyses evaluate how sensitive the operating limits of equipment are to the variation of each key operating parameter.

Managing Dynamic Loads in Equipment Analysis

Machinery, such as pumps, compressors, fans, blowers and conveyors, are under dynamic operating loads. Some equipment, including chutes, hoppers, bins and feeders, may also be under shock and impact loads from the discharged bulk materials. And the fixed equipment and piping downstream of machinery, such as pumps and compressors, can be under dynamic loads imposed by the machines.

Determine dynamic loads using either simulation of the characteristics of the operations or by an estimation method. A common estimation practice is to apply a dynamic factor to the static loads. The value of the dynamic factor depends on many parameters, such as the velocity of operation/load, the acceleration and the fundamental natural frequency of the system under the loading. Charts and tables are available in textbooks, codes, standards and even online for estimating dynamic factors based on the details of the loading, operation and equipment.

For machinery and rotating equipment without a soft starter or soft control, the experienced shock and dynamic effects at the transient operating cases are relatively high. An example includes the dynamic/transient loadings upon start up.

Shock and dynamic effects are relatively low when the driver is equipped with a variable speed control or similar soft-control provisions. The dynamic loads and dynamic factors have been key considerations for many uprating and rerating assessments.

Addressing Uncertainties and Material Variations

In reality, the strength and capability of equipment depends on various sources of uncertainties, including varying operational parameters, deviations in operation, corrosion levels, material properties, defects, imperfections (such as initial geometric imperfections), properties of parts and pieces, types and details of loadings, residual stresses and loss of control in operation.

The yield strength or stress is one of the most important characteristics of equipment, which often has a significant influence on the load-carrying and pressure-carrying capacity of the equipment.

Uncertainties and variations associated with corrosion, yield strength or stress, tensile strength and behavior of materials should be handled with care. In other words, these uncertainties and variations should be considered when determining operating limits and life of equipment and machinery.

Conservative Design Philosophy in Traditional Equipment

In the past, equipment OEMs applied conservative rules to design their machinery for reliable and safe operation and a long lifespan. In many cases, they applied these tight limitations due to the uncertainties and indeterminacies involved.

They often used simple sizing rules and oversimplified formulas with large safety factors rather than detailed operational assessments. Modern design procedures recommend detailed assessments and simulations of operating situations and predictions of all possible failures, such as all possible over-pressure, over-stress, buckling and fatigue.

For a long operating life and high reliability, stresses and deformations in the equipment should ideally be very low. For strong machinery under normal operating conditions intended for unlimited life, typically stresses and deformations are far less than allowable values specified in codes and standards. As a set of very rough indications:

Stresses might be below 0.22 × yield strength or even below 0.18 × yield strength.

Deformations or deflections might be less than 1/1500 length or height — for instance, less than 2 mm for an equipment with length of 3 m.

These values are far below the commonly specified limits in different textbooks, practices, codes, and standards. Obviously, higher safety factors are needed where the service is risky, dangerous, explosive or toxic. When personnel are involved in the operation, more stringent requirements have been specified and met.

A common specification could be fail-safe requirements, such as allowing for the simultaneous malfunction of two items. Although more equipment today is intended for remote operation, there are some exceptions. For example, when operators are at risk, adequate provisions and controls should be provided for their safety. Equipment with platforms and ladders requires more stringent safety factors than equipment that doesn’t have such access points or is remotely located with minimal risk of people being present nearby.