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.
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.
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.