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CPI Machinery: Commissioning, Startup and Piping

By Amin Almasi, WorleyParsons Services Pty. Ltd. |

When it comes to rotating machinery (pumps, compressors, gas turbines, steam turbines, turbo-expanders, and others) operating in the chemical process industries (CPI), installation, piping, support, pre-commissioning, commissioning and startup play crucial roles. Machinery maintenance and repair can be quite costly, and perhaps because of shortages of skilled manpower, particularly at remote site locations, CPI plants have given less attention to technical details for machinery installation, piping and commissioning.

When one considers that the initial investment costs for a rotating machine can be in range of $200–2,000/kW ($0.2–2 million/MW), whereas daily production loss can exceed $1–2 million for many modern (and large) CPI plants, one can see that it is necessary to protect these machines by doing an adequate installation, piping, commissioning and startup job.

 Figure 1. Shown here are examples of centrifugal compressor impeller assemblies (rotor assemblies) ready for shop balancing

 

Machinery piping design

Although there are rules and practices adopted for the design of general CPI plant-piping in order to facilitate the design and to avoid common errors, there are usually some special rules or exceptions when it comes to piping associated with CPI rotating machines; failure to properly handle these exceptions and special rules can cause difficulties in the installation and create problems during the operation of the rotating equipment.

The piping between a rotating machine and suction or discharge equipment (such as suction or discharge vessel or cooler) is critical, particularly when the operating temperature is very different than ambient temperature, and when the piping sizes are large. Sometimes both the rotating machine and the peripheral equipment are located on the same side of the pipe-rack. In that case the connecting pipe is supported on the pipe-rack and during operation, and because of the thermal difference, the piping generally moves away from both pieces of equipment. This layout and piping design offers relatively smaller loads on the equipment nozzles.

In some CPI units where space is constrained, the rotating machine and the equipment (suction or discharge equipment) are situated on opposite sides of the pipe-rack. In this case, the center point of the line running on the pipe-rack acts like a pivot point. The connecting line on both sides of this center point will move toward the equipment. This arrangement induces marginally more loading on the equipment nozzle compared to having equipment on the same side of the pipe-rack.

In many CPI plants, the discharge of a rotating machine (compressor, pump or others) is usually routed to an air-cooler or a heat exchanger (intercooler or after-cooler). Usually the rotating machine is offset from the air-cooler (or the heat exchanger). In this case, the vertical thermal expansion is absorbed by the deflection of the horizontal legs and similarly, the thermal expansion of the horizontal legs is absorbed by the deflection of the vertical legs. Another arrangement is when the machine and the air-cooler (or the heat exchanger) are located in the same line and relatively close to each other. In such case, it is necessary to provide enough flexibility to absorb the thermal movements. Loop(s) should be provided. To support a loop, it should be routed on a nearby pipe-rack (if possible), or else a proper structural support should be provided. The offset location of the machine to the air-cooler (or the heat exchanger) provides better piping flexibility.

Steam turbines or turbo-expander turbines (whether cryogenic or hot-gas type) are strain sensitive and operate at very high temperature difference with relatively low allowable nozzle loads. Usually, numerous layout details should be considered to provide the maximum possible flexibility. Even a good layout can create a problem if proper supporting is not considered. Good recommendations for the turbine piping are as follows:

1. A flexible piping arrangement may encircle the turbine.

2. Line stops may be located in such a way that they match the turbine centerlines. This line stop arrangement can nullify the thermal movement effects and help in reducing the nozzle loads.

3. Supports near turbine nozzles should have low friction. Spring supports with minimum variability are recommended. They can be supported with hanger rods and rigid struts that reduce the friction at the support.

For parallel rotating machines (two or more identical machines working in parallel), the piping of each two identical trains could be mirror imaged in order to get a common maintenance area (an easy access).

 

Piping system vibration

The most obvious effect of piping vibration or pulsation — particularly sustained vibrations — is fatigue failure, especially at critical high-bending stress regions. The existence of such vibration suggests three approaches:

• Provide mechanical restraints to prevent the movement of the piping system (a damping solution)

• Elimination or control the vibrations or pulsations at their source (particularly those from rotating machines)

• Eliminate the coupling. Elimination of the transmission routes that excite the piping system vibrations or pulsations (for example, eliminate the excitations by changing the natural frequency of the system to avoid a resonance)

While each of these three approaches can be valuable, no single one will necessarily be the optimum for all cases of the rotating machine piping. Some of them could be excessively expensive, impossible, impractical or ineffective for a specific case. For example, the cost of mechanically restraining a large-size overhead piping could be very high.

Many criteria have evolved for the control of piping vibration or pulsation (from different points of view such as noise, safety, environmental and others). The fact is, the amplitude of the vibration or pulsation is not the biggest problem. The real danger is the dynamic stress level in the piping system, and this stress should be limited to below the destructive levels. Whenever the vibratory stress exceeds the endurance level of the piping, a failure can occur. By this argument, any vibration-amplitude criteria for CPI piping system should be applied very carefully. Special considerations should be given to the configuration of the piping system being studied. In other words, vibration amplitude criteria for a piping system should consider the piping configuration involved. The stress level in a piping configuration is mainly a function of the physical distortion.

The dynamic stress generated in a cantilever span due to a certain vibratory deflection is different than in that generated in another complex configuration by an equal vibratory deflection. A lower stress is generated in a long (straight) piping span compared to a short complex one (such as “L” shape, “U” shape, loop shape and similar) as a result of the same dynamic deflection. An investigation into the dynamics of different piping configurations shows that the variation in the dynamic stress per unit of vibratory deflection tracks rather directly with the natural frequency for a given piping. For example, a long (straight) flexible span has relatively low stress and a relatively low natural frequency. This natural frequency variation could be used to normalize allowable dynamic stress criteria (of course, it is just an approximation).

Based on this approach, the vibration velocity criteria can be specified for each piping configuration. As a rule of thumb, the vibration velocity limit can be stated as 6–9 mm/s for a long piping span, and 3–6 mm/s for a short complex piping (such as “L” shape, “U” shape, loop shape and others). These values are just rough estimates, and higher vibration values may be acceptable for some cases.

 

Piping support and flexibility

Piping-support systems are generally designed according to two major rules. First, the support locations are determined by the guidance of the maximum allowable spans. Secondly, the support types are selected based on the expected thermal displacement, dynamic situations, stress analysis and similar operational effects.

Flexibility studies (or a piping stress analysis) are intended to verify that stresses in CPI plant piping and the loads on the equipment nozzles (forces and moments at the fixed equipment or rotating machine nozzles) are within the acceptable limits through all anticipated phases of normal and abnormal operation. All possible situations, including the installation at ambient temperature, various operating conditions, the startup, the normal shutdowns, the emergency shutdowns, upsets and others, should be considered.

The piping of rotating equipment should be designed and supported so that equipment can be dismantled or removed without adding temporary supports (that means, by only removing spool pieces).

Rotating machine piping should be supported on integral extensions of the equipment support structure or independent structure (or support), and not be anchored to the equipment or its base-plate.

 

Alignment

Improper machinery alignment can cause excessive vibration, premature wear and early failure. Proper alignment can greatly improve bearing and seal life, reduce vibrations and boost reliability and overall performance. Adequate clearance for each machine casing (for example, driver, gear unit and driven equipment) is important to permit a proper alignment. Some authorities recommend the rotating-machine shaft-interface fit (the alignment tolerance) with a tolerance of around 0.0005 times of the shaft diameter (approximately 0.01–0.02 mm for typical 25–50 mm shaft diameters). Some textbooks recommended an alignment tolerance of around 0.01 mm overall, regardless of the shaft diameter. Special rotating-machine trains may need tighter alignment tolerances. Some ordinary machinery trains with flexible couplings (such as flexible couplings that transmit torque through elastomeric materials) may tolerate higher interface fits than those mentioned above. The coupling spacer length is also important since the parallel misalignment accommodation is directly proportional to that length.

Alignment tolerances given by coupling manufacturers may perhaps be only true for the coupling itself and could be excessive for coupled rotating machines.

The real criterion for alignment are the vibrations from the machinery when running. If excessive, particularly at twice the running speed (or axially), further alignment improvement is required. Analysis of failed components such as bearings, couplings and seals can be used to indicate the need for an improved alignment. Commonly-used alignment methods fall into three broad categories: reverse-indicator; face-and-rim; and face-face-distance.

Reverse-indicator. The reverse-indicator is the preferred setup for aligning modern CPI rotating machinery. The accuracy of this method cannot be affected by the axial movement of the shafts in sleeve bearings (hydrodynamic bearings). Both shafts should turn together (generally both shafts should be rotatable and coupled together), so coupling eccentricity or surface irregularities do not reduce the accuracy of the alignment readings. Geometrical accuracy is usually better using this method, compared to other methods.

This method is very convenient and generally implemented without disconnecting the coupling. For complex alignment situations, where thermal expansion or multi-casing trains are involved, the reverse-indicator method could be used quite readily. Usually, single-axis leveling is sufficient for machines using rolling-element-bearings, and two-axis leveling could suffice for machines employing sleeve-bearings.

There are some limitations for the reverse-indicator alignment method. If the coupling diameter exceeds the available axial-measurement span, the geometrical accuracy could be poor using the reverse-indicator alignment method compared to other methods.(such as the face-and-rim). Nowadays, the general trend is toward high-torsional-stiffness couplings (metallic, flexible spacer-type couplings), and the reverse-indicator method is nearly always the selected alignment setup for modern CPI installations.

Face-and-rim. The traditional face-and-rim method was popular decades ago. It can be used on large and heavy rotating machines whose shafts cannot be turned (of course, some run-out error may occur due to shaft or coupling eccentricity). It may offer a better geometrical accuracy than the reverse-indicator method for couplings with short spans (a small span-to-diameter ratio). Generally, this method is better and easier to apply on short coupling spans (or small non-critical machines). If this method is used on a machine with sleeve bearings, the axial float error could be significant, and a special procedure is usually required. As a general rule, a two- or three-axis leveling is required for rolling-element bearings and sleeve bearings, respectively (The reverse-indicator method requires leveling in one less axis for each). For long spans, this method requires spacer removal to permit the face mounting.

Face-face-distance. The face-face-distance alignment method is introduced for very long spans (such as trains that use a long transmission shaft instead of a coupling). This method is usable without an elaborate long-span bracket or other special considerations. The geometrical accuracy of this method is normally lower than the other two methods. It has no advantage over other commonly-used alignment methods for anything except the long spans (the long connecting shafts).

Laser alignment. Nowadays, methods using laser optic alignment have become very popular. Devices usually use a semiconductor emitting a laser beam in the infrared (IR) range (the wavelength around 800 mm) along with a beam-finder incorporating an IR detector. Physical contact is not required (it is replaced by the laser beam). Typical accuracy is one micron using the modern laser alignment methods. With the data automatically obtained from the sensor, the system can instantaneously yield the horizontal and vertical adjustments required for the alignment of the machine.

Many machinists make alignment corrective movements by trial and error (some may spend one or two days aligning a machine this way). However, by knowing how to calculate the corrections or using an advanced laser-lignment module, the time could be cut to two hours or less.

The thermal expansion (or contraction) of machines can often be significant for the alignment purposes (depending on the machine configuration). The movement of one machine casing relative to others is the main concern (absolute movements cannot affect the alignment). Movements caused by the pipe loads, the fluid forces and the torque reactions usually have important effects. The vibration can give an indication of whether thermal movements or other operational effects are causing misalignment problems during the startup or during operation. It may be necessary to consider the thermal-operational movement correction in the machinery alignment during the commissioning. One of the best methods can be mechanical measurements on a machine during the operation at the site with the job foundation and the final piping. Another useful recommendation is to make the machine and piping adjustments while the machine is operating, using vibration measurements as the primary reference.

 Figure 2. This photo shows a typical piping arrangement for a pumping system

 

Balancing

Imbalance can be caused by a variety of reasons, including tolerances in fabrication (or assembly), variation within materials (such as voids, porosity or similar), any non-symmetry, distortion, deflection, dimensional changes, degradation, and other manufacturing or operational problems. Manufacturing processes are the major source of imbalance. However, improper shipment, assembly, installation and commissioning can also lead to imbalance of the rotating assembly.

Often, field balancing is required at the CPI site during commissioning or startup. If the danger of this imbalance vibration is not recognized, costly damages could occur after a very short operation time. This may result in the destruction of the bearing, seal damage, cracks in various components, foundation damage, mounting system problem or other issues.

With a simply supported rotor assembly (bearings at both ends), vibrations due to the imbalance will be mainly in the radial plane. In the case of an over-hung rotor, high axial vibrations may also occur (the amplitude of axial vibrations could be comparable to those measured radially).

An unbalanced rotor assembly can cause high stresses in the rotor itself, in its support structures and in the entire machine-foundation system. Balancing of the rotor may be necessary to increase the bearing and seal life, minimize the vibration (and the stresses), minimize the noise, minimize the effects and risks of fatigue, minimize the power losses and increase safety. Imbalance can be of four basic types: static, couple, quasi-static and dynamic.

Static. Static imbalance exists when the principal axis of inertia is displaced parallel to the shaft axis. With a statically unbalanced rotor, the amplitude and phase of the vibration at both ends of the rotor are the same. This type of imbalance is found primarily in narrow, disc-shaped rotating parts such as flywheels or machine wheels (for example, thin impellers). It can be corrected by a single mass correction. Static balancing is satisfactory only for relatively slow-revolving, disc-shaped components or for parts that are subsequently assembled onto a larger rotor that is then balanced dynamically as an assembly.

Couple. Couple imbalance arises when two equal unbalanced masses are positioned at opposite ends of a rotor and oriented 180 deg. from each other. A couple imbalance needs another couple to correct it.

Quasi-static. The quasi-static imbalance represents the specific combination of static and couple imbalance where the angular position of one couple component coincides with the angular position of the static imbalance.

Dynamic. In the case of a dynamic imbalance, the central principal axis of inertia is neither parallel to, nor intersects the shaft axis. It is the most frequently occurring type of imbalance and can only be corrected by the mass corrections in at least two planes perpendicular to the shaft axis.

If the rotor support system is rigid, the unbalanced dynamic force is usually larger than if the rotor support system is flexible (except at the resonance). In practice, rotor support structures are neither entirely rigid nor entirely flexible, but somewhere in between.

A field-balancing package usually provides sensing and monitoring instrumentation needed to measure the balancing of a rotor while the rotor runs inside the machine at the site (in its own bearings and under its own power). Basically a field-balancing system consists of combinations of proper transducers and measurement devices that provide an imbalance indication proportional to the vibration magnitude. A suitable calculation module is used to convert the readings (usually the vibration, in several runs with the test masses) into the magnitude and phase angle of the required correction masses. The vibration measurements at one end of a machine could be affected by the imbalance vibration from the other end.

To determine accurately the size and the phase angle of needed correction masses, at least three runs are required. One to identify the “current” condition, the second with a test mass in one plane and the third with a test mass in anther correction plane.

Nozzle flange connection

To minimize loads on the machinery nozzle and facilitate the installation of piping, the rotating machine nozzle flanges should be parallel to the plane shown on the machinery drawing to within 0.1–0.5 deg. (depending on the equipment details). It is very important to correctly align flanged joints (the equipment nozzle flange and the piping flange). Flanges that are bolted up unevenly, in extreme cases, can cause some bolts to be nearly loose, while others are so heavily loaded that they locally crush the gasket. This can lead to leakage.

Flanges are designed to accommodate specific sizes and types of gaskets. For a given bolt load, a narrower gasket will experience a greater unit load than a wide gasket. A gasket with proper type, sizes and width should be employed. The gasket thickness determines its compressibility and the load required to seat it. The thicker the gasket, the lower the load necessary for seating. A proper thickness assures sufficient compressibility to accommodate slight facing irregularities while having a sufficiently high seating load. For a stud larger than 1 in. (25 mm), a proper hydraulic wrench should be used.

Bolting at operating conditions during the startup and the operation runs can be an important factor in minimizing flange-connection problems. During operation, the temperature change can cause the bolts and gaskets to deform permanently, which may cause a loss of bolt stress. Bolting at the operating condition helps correct this effect. The objective is to restore the original bolt stress that has dropped (or changed) due to yielding or creep of the flanged joint components. The bolting procedure should start at the point of the relative peak of the gap and proceed in a crisscross pattern.

 

Inlet strainer

It is very important to start up any CPI rotating machine with clean piping, particularly on the suction side. Any dirt, rust, welding beads or scale that is carried to the machine can cause serious problems. Even though cleaning procedures have been carefully followed, a temporary strainer should be installed in the suction line of each CPI rotating machine casing. Provisions should be made in the piping to check the pressure drop across the strainer and to remove the strainer element for cleaning. The machinery CPI piping should be fabricated with sufficient flange joints so that the piping, the permanent filters and the temporary strainers can be dismantled easily for cleaning. It is better to clean piping in sections before the actual erection.

 

Installation and commissioning

Electrical equipment. Electric machine problems appear in two major forms: 1) Mechanical, particularly rotor or bearing difficulties; and 2) Electrical, mainly electric winding or electrical-system problems.

Usually, a large portion of reported electric-machine issues are bearing problems. Other common electric-machine problems are misalignment, lubrication problems, excessive shaft loadings and environmental issues (ice, snow, sand or dirt).

There can be operational problems if the center of gravity of the electric-machine rotor assembly is different than the magnetic center. In this condition, the rotor continually hunts for the position it wants to run in. This can manifest itself in high axial vibrations. The phase and frequency of these vibrations may or may not prove to be synchronous. A proper magnetic alignment is required to solve this issue.

If dampness in the insulation of the electric machine is suspected as the result of shipment, storage or installation, the insulation resistance of the windings or other electrical components should be measured before the site commissioning.

Lubrication system. The cleanliness of a lubrication system used in a CPI plant is extremely important. When flushing at the site, the same lubrication oil should be used for flushing as that specified for operation. Any dirt or debris in a lubrication system should be collected at the lubrication filter (or additional strainers) during the flushing. Sufficient time is needed for a proper flushing (usually several days).

The cleaning capacity of the lubrication oil is better at relatively high oil velocities, a relatively high temperature and low viscosity. The transportation capacity of the lubrication oil is best when the oil is relatively thick at a relatively low temperature and high viscosity. The temperature of the lubrication oil should be varied within the specified low and high limits, several times, to achieve a proper flushing of the lubrication system.

 

Seal system commissioning

During the commissioning and start-up of a rotating machine, the damages of mechanical seals occur particularly often. Modern mechanical seals are usually supplied as cartridge units (pre-mounted) and most often do not require adjustment during the machine assembly, installation or commissioning. Increased imperfections of concentricity and run-out in the shaft can lead to high vibrations and decrease significantly the service life of the seal. Stainless-steel pipes should be used for entire seal services with sufficient cross-section to ensure that the mechanical seal can be supplied with required clean seal fluid at any operation situation.

Dry-gas seal system. The warning limit for a dry-gas seal leakage is often five times the expected (normal) leakage value. The leakage should be carefully controlled during the startup and the initial period of the operation.

During operation, the dry gas seal needs a positive differential pressure (usually minimum 1–3 bars) in order to provide for a sufficient cooling of the dry gas seal. The commissioning or startup with too-low differential pressures may lead to damage of the dry gas seal by means of overheating, hang-up, wear or contacting sliding faces. The pressure change rate should usually be limited for reasons of operational safety (often in the range of 10–20 bars/min). The supply of the bearing lubrication oil should only be started when the bearing sealing system is provided with the separation gas (usually nitrogen gas). The operation of the dry gas seal requires minimum values of the rotating machine speed. If the machine does not rotate, a minimum differential pressure is required to ensure that the sliding faces lift off statically. In the case of rotating machine trains with very long startup or shutdown times, the operation with contact (contact of the seal faces) during every start/stop cycle has to be expected, depending on the operating parameters. This is an important consideration in the startup.

 

Operation and maintenance

The foundation and mounting systems beneath rotating equipment will often deteriorate over time. When this happens, the dynamic forces or deflections in the system can go beyond permitted levels. Very costly foundation (or mounting) system repair and re-grouting may be implemented, if downtime can be accepted in the CPI plant. Successful alternative approaches to the repair (or renewal) of mounting systems (such as renovation methods based on “shimming”) without requiring downtime are also available and have recently become very popular.

For many rotating machines, the tight manufacturing clearances and complex geometry make refurbishing difficult in a case of major damage or catastrophic component failure, such as a bearing failure. As another example, the rotor may end up digging and melting into the casing. If bent, the rotor should be replaced. If not bent, it may be refurbished, which is usually a difficult process. For example, the rotor’s sealing edges can often be sprayed in case of damage. Repair of the machines casings is both difficult and risky — for machining and even more for welding.

Edited by Gerald Ondrey

 

 

Author

Amin Almasi is lead rotating equipment engineer at WorleyParsons Services Pty Ltd, Brisbane, Australia. He previously worked at Technicas Reunidas (Madrid, Spain) and Fluor (various offices). He holds chartered professional engineer license from Engineers Australia (MIEAust CPEng – Mechanical), chartered engineer certificate from IMechE (CEng MIMechE), RPEQ (Registered Professional Engineer in Queensland) and he also holds M.S. and B.S. degrees in mechanical engineering. He specializes in rotating machines including centrifugal, screw and reciprocating compressors, gas and steam turbines, pumps, condition monitoring and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME, Vibration Institute, SPE, IEEE, and IDGTE. He has authored more than 60 papers and articles dealing with rotating machines.

 

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