Gene Vogel
EASA Pump and Vibration Specialist
A quality repair in an EASA service center will yield a motor or pump that will meet just about any vibration specification. And good EASA technicians could use their “finger vibrometer” to verify that. However, a large number of EASA customers demand a more formal means of verifying acceptably smooth operation.
A few sophisticated customers have developed their own vibration standards, but most depend on standards organizations such as the National Electrical Manufacturers Association, or NEMA (electric motors), the Hydraulic Institute (HI) (pumps) and the International Organization for Standardization (ISO). There are also industry specific standards organizations such as the American Petroleum Institute (API) for refineries and the Submersible Wastewater Pump Association (SWPA). A working knowledge of the various standards will be beneficial for EASA service center technicians and managers.
The three examples in this article will help illustrate the scope and diversity of the various vibration standards a service center may encounter.
Criteria
One of the more confusing aspects of the various vibration tolerances is that each standard uses different criteria to assign acceptable levels. NEMA standards categorize motors by “machine type” but ISO designates different levels for “large rigid foundations” and for “large soft foundations.” API provides tolerances based on flow rate while the HI tolerance is by input power. Just knowing what information you need is a challenge in itself. The criteria used by the various standards are discussed below.
HI lists eleven different pump configurations (excluding submersibles) and provides sketches that clarify the identifying characteristics of each. For each type, vibration velocity (in/sec, mm/sec) limits are provided based on input power. Submersibles are designated by mounting method with similar limits based on input power. HI standards for submersibles were developed with cooperation from the SWPA, which also references the same standard.
API separates vertical pumps from those that are mounted horizontally. Horizontal pumps are further segregated by speed and absorbed power per stage, above or below 3600 rpm and 300 kW (400 hp)/stage. Absorbed power is based on capacity and head rather than input power. The smaller horizontal group and the vertical group each have a single vibration velocity limit assigned.
Pumps over 3600 rpm or 300 kW (400 hp)/stage have vibration velocity limits based on both speed and power.
NEMA provides no-load vibration standards for standard industrial motors, but excludes motors connected to loads. These motor-only standards consist of vibration velocity limits based on the frequency of the vibration and the machine type. Machine types are described in general terms by application.
ISO standards are provided for general machinery and specifically for rotodynamic pumps. Centrifugal pumps are rotodynamic pumps. The general machinery standard initially provided a table of “interim” limits which are superceded by specific limits for general machinery and specific limits for rotodynamic pumps.
The limits for general machinery classify machines by size and by rigid or flexible mounting. The size classification is either above or below 300 kW (400 hp) or above or below shaft height of 315 mm (12.4”). Limits are provided in both velocity units and in displacement units. The limits for rotodynamic pumps categorize pumps by critical or non critical, and further by above or below 200 kW. These limits are provided in velocity units. Also, pumps operating below 600 rpm have different limits provided in displacement units. The limits specified for rotodynamic pumps apply to close coupled pumps and motors, since the motor is integral to the pump. But these limits do not apply to separate motors. So a motor coupled to a pump will have different limits for the motor than for the pump.
Submersible pumps
Separate vibration limits for submersible pumps are provided only by HI. API and ISO apply the same limits as base mounted pumps. HI specifies a single transducer mounting at the top bearing, 45 degrees radial from the discharge nozzle. The limit is then based on the input power (bhp) 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 in/sec pk is allowed for submersibles with single vane impellers.
Other considerations
When comparing vibration limits from any of the standards, care should be taken to observe whether limits are in rms, peak (pk) or peak-to-peak (pkpk) units. HI lists all limits in rms units. These can be converted to the more common pk units simply by multiplying by v2, or to pkpk by multiplying by 2v2. The API lists limits for velocity in rms, but units of displacement are already in pkpk. ISO limits are in rms units.
The API vibration limits actually define flow and head “preferred” operating conditions. Since the 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 which is 30% greater than 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 to either overall vibration amplitude, or to individual vibration frequencies as would be indicated in the vibration spectrum. So a motor having vibration of 0.1 in/sec pk at a rotating speed of 1795 CPM plus 0.1 in/sec pk at 7200 CPM (2x line frequency), would still meet the specification, even though the overall vibration level would be above 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 vibration limits for shaft vibration for those machines with hydrodynamic (sleeve) bearings. Usually these are applied to machines fitted with proximity probes.
There are also standards for rotor balancing which are much different than the pump and machinery vibration standards discussed above. Balancing standards prescribe the allowable unbalance in units of oz-in or g-mm per unit of rotor weight. These standards do not say anything about the vibration level of machines when assembled and running.
Conclusion
When customers provide vibration limits which must be met for new or repaired pumps and motors, it is useful to know the source of the vibration limits. Often only a numeric limit is provided, without any of the qualifying criteria. This can be particularly troublesome when a machine is reported to not “meet specifications” after it is installed. Knowing the criteria that should apply to the specification can be very helpful in discovering why the vibration level appears to exceed the limit. Vibration limits provided by customers may be misapplied, or operating conditions may be different than those prescribed by the applicable standards organization.
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