Working knowledge of standards improves communication with service providers.
A quality repair by a professional service center will yield a motor or pump that will meet just about any vibration specification. Good repair technicians could verify that with a "finger vibrometer." Most maintenance professionals, however, demand a more formal means of verifying acceptably smooth operation.
A few sophisticated plant engineers have developed their own vibration specifications, but most of them reference standards developed by such organizations as the National Electrical Manufacturers Association (NEMA) for electric motors, the Hydraulic Institute (HI) for pumps and the International Organization for Standardization (ISO) for motors and pumps. Various industry-specific organizations such as the American Petroleum Institute (API) for refineries and the Submersible Wastewater Pump Association (SWPA) publish vibration standards as well.
Given the scope and diversity of these vibration standards (three examples of which are given below), it's not surprising that sometimes new or repaired pumps or motors appear not to meet the specifications supplied by the customer. Although miscommunication is usually the root cause, such situations typically arise when qualifying criteria are omitted, or the applicable standards are not well understood or are misapplied–for example, the operating conditions are different than prescribed in the referenced standard. A working knowledge of the major vibration standards will, therefore, benefit anyone who specifies motor and pump purchases or repairs, as well as those who require vibration analysis services.
Criteria
One of the more confusing aspects of vibration tolerances is that each standard uses different criteria to assign acceptable levels. NEMA Standards Publication MG 1: Motors and Generators, for example, sets vibration limits for motors by machine type, whereas ISO standards designate different vibration levels for large rigid foundations and large soft foundations. API bases its tolerances on flow rate, while HI defines them in terms of input power. Just knowing what information you need is a challenge, so briefly reviewing the criteria used by the standards should help.
- HI standards—ANSI/HI Pump Standards list 11 different pump configurations (excluding submersibles) and illustrate the identifying characteristics of each. The vibration velocity limits (inches per second, millimeters per second) for each configuration are based on input power. Submersibles, which are designated by the mounting method, have similar vibration limits that are also based on input power. HI standards for submersibles were developed with cooperation from SWPA, which also references these standards.
- API standards—API standards separate vertical pumps from those that are mounted horizontally. Horizontal pumps are further segregated by speed and absorbed power per stage—above or below 3,600 rpm and 300 kilowatts (400 horsepower) per stage. Absorbed power is based on capacity and head rather than input power. The smaller horizontal group and the vertical group are each assigned a single vibration velocity limit. Pumps over 3,600 rpm or 300 kilowatts (400 horsepower) per stage have vibration velocity limits based on both speed and power.
- NEMA standards—NEMA MG 1 provides no-load vibration levels for standard industrial motors but excludes motors connected to loads. These motor-only standards base vibration velocity limits on the frequency of the vibration and machine type. MG 1 describes machine types in general terms by application.
- ISO standards—ISO publishes standards for general machinery and specifically for rotodynamic (i.e., centrifugal) pumps. The general machinery standard initially included a table of interim vibration limits. This has been superseded by specific limits for general machinery and specific limits for rotodynamic pumps.
The ISO standard provides vibration limits for general machinery in velocity and displacement units, based on machine size (above or below 300 kilowatts/400 horsepower, or above or below shaft height of 315 millimeters /12.4 inches) and mounting (rigid or flexible).
For most rotodynamic pumps, vibration limits are given in velocity units, based on pump size (above or below 200 kilowatts/268 horsepower) and classification (critical or non-critical). Pumps operating below 600 rpm, however, have different vibration limits that are given in displacement units.
ISO vibration limits for rotodynamic pumps apply to close-coupled pumps and motors (since the motor is integral to the pump) but not to separate motors. Therefore, a motor that is coupled to a pump will have different vibration limits than the pump.
Submersible Pumps
Only HI standards provide separate vibration limits for submersible pumps; API and ISO apply the same limits for base-mounted pumps. HI specifies a single transducer mounting at the top bearing (45 degrees radial from the discharge nozzle) and bases the vibration limit on the input power (brake horsepower) and the mounting method. Submersibles that mount on the discharge flange (rail-mounted) are allowed a slightly higher limit than floor-mounted pumps. A further exception of an additional 0.14 inches/second (pk) is allowed for submersibles with single-vane impellers.
Other Considerations
When comparing vibration limits from any of the standards, be sure to notice whether the units are root mean square (rms), peak (pk) or peak-to-peak (pkpk). HI lists all limits in rms units. These can be converted to the more common pk unit by multiplying by √2, or to pkpk by multiplying by 2√2. API gives velocity limits in rms units, but units of displacement are already in pkpk. ISO limits are in rms units.
The API vibration limits actually define preferred operating conditions for flow and head. Since vibration increases as flow rates increase or decrease from the best efficiency point (BEP), the preferred operating region is the span of flow rates near the BEP where vibration is below the limit. A second, allowable operating region is defined by a vibration level that is 30 percent greater than that in the preferred region. This approach infers that increased vibration is the result of flow-induced hydraulic forces, not mechanical faults. It also applies only to pumps running under normal (or simulated normal) flow and head conditions.
NEMA vibration limits can be applied either to overall vibration amplitude or to individual vibration frequencies as would be indicated in the vibration spectrum. Therefore, a motor having vibration of 0.1 inch per second (pk) at a rotating speed frequency of 1,795 cpm, plus 0.1 inch per second (pk) at 7,200 cpm (2 x line frequency), would still meet the specification, even though the overall vibration level would exceed the limit. Conversely, if the overall reading is below the limit, the amplitude at all individual frequencies must also be below the limit.
Several of the standards organizations also provide shaft vibration limits for machines with hydrodynamic (sleeve) bearings. Usually these are applied to machines fitted with proximity probes.
There are also standards for rotor balancing that are much different than the pump and machinery vibration standards discussed in the previous section. Balancing standards prescribe the allowable unbalance in units of ounce to inch or grams to millimeters per unit of rotor weight. These standards do not say anything about the vibration level of machines when assembled and running.
Conclusion
Providing vibration specifications for new or repaired pumps and motors is an excellent way to communicate expectations to a service center or other supplier. If done correctly, it also provides a yardstick for measuring the results. To achieve that outcome, avoid giving only a numeric vibration limit. Instead, review the source standard to make sure it applies, and then reference it in the specification along with any qualifying criteria. Armed with this information, professional service centers often spot problems that could arise if the vibration limits have been misapplied or the operating conditions are different than prescribed in the applicable standard. This information can also be helpful in discovering why the vibration level appears to exceed the limit should a machine be reported to not "meet specifications" after it is installed.
Pump Standards Example 1
Horizontal end suction pump
30 horsepower 1,775 rpm
150-foot head (65 psi, water), 1,200 gallons per minute
Motor shaft height—7 inches
Mounting—rigid
New installation—non-critical
Pump Standards Example 2
Vertical lift station pump
300 horsepower, 1,780 rpm
85-foot head (37 psi, water), 7,500 gallons per minute
Equivalent motor shaft height—11 inches
Mounting—flexible
In-service—non-critical
Pump Standards Example 3
Vertical turbine pump
75 horsepower, 440 rpm
350-foot head (150 psi, water), 1,250 gallons per minute
Equivalent motor shaft height—20 inches
Mounting—flexible
In-service—critical
Pumps & Systems, June 2011