Maximum continuous speed

Based on the chosen standard, the shaft was originally designed for a maximum continuous speed of 52,000 rpm. This maximum continuous speed was limited by ... 

Maximum continuous speed for generator drive duty is synchronous speed. 

Trip speed is the speed at which an independent emergency device is activated and sends a signal to close the expander inlet butterfly valve system. This is approximately 105% of maximum continuous speed or synchronous speed (whichever is higher). 

Overspeed is the speed at which the expander rotor is tested to prove its design capability for this service. Overspeed is 115% of maximum continuous speed. 

A requirement of the standards is that the fundamental natural frequency of the blade should be at least two times the maximum continuous speed, and at least 10% away from the passing frequencies of any stationary parts. Experience has shown that the natural frequency should be at least four times the maximum continuous speed. Care should be exercised on units where there is a great change in the number of blades between stages. 

Maximum continuous speed, 105 percent = Initial (lesser) speed at. 707 x peak amplitude (critical) Wg = Final (greater) speed at.707 x peak amplitude (critical) W2-/v,= Peak width at the half-power" point... 

The turbine undergoes three basic tests, these are hydrostatic, mechanical, and performance. Hydrostatic tests are to be conducted on pressure-containing parts with water at least one-and-a-half times the maximum operating pressure. The mechanical run tests are to be conducted for at least a period of four hours at maximum continuous speed. This test is usually done at no-load conditions. It checks out the bearing performance and vibration levels as well as overall mechanical operability. It is suggested that the user have a representative at this test to tape record as much of the data as possible. The data are helpful in further evaluation of the unit or can be used as base-line data. Performance tests should be conducted at maximum power with normal fuel composition. The tests should be conducted in accordance with ASME PTC-22, which is described in more detail in Chapter 20. 

Gear elements must be multiplane and dynamically balanced. Where keys are used in couplings, half keys must be in place. The maximum allowable unbalanced force at maximum continuous speed should not exceed 10% of static weight load on the journal. The maximum allowable residual unbalance in the plane of each journal is calculated using the following relationship... 

As with the motor driver, the steam turbine must be matched to the compressor. Also, a turbine rating of 110% of the maximum power required by the compressor should be specified. This should be at the compressor s normal speed point. The turbine speed should include a maximum continuous speed 105% of the normal compressor speed. API Standards 611 and 612 cover general purpose and special purpose steam turbines 7, 8]. 

N = operating speed, alternatively maximum continuous speed, rpm... 

Obviously this test applies only to the centrifugal compressor. The manufacturer normally performs this test whether specified or not. Since the user does have an interest in the test, some discussion of the parameters is in order. API 617 mandates the test, which includes the requirement to spin the impeller at a speed of 115% of the maximum continuous speed for a duration of one minute. 

The vibration characteristics, determined by use of the instrumentation, will serve as the basis for acceptance or rejection of the machine. API standards generally require that the equipment be operated at speed increments of approximately 10% from zero to the maximum continuous speed and run at the maximum continuous speed until bearings, lube-oil temperatures, and shaft vibrations have stabilized. Next, the speed should be increased to trip speed and the equipment run for a minimum of 15 minutes. Finally, the speed should be reduced to the maximum continuous speed and the equipment should be run for four hours. API does not require that the four hours be uninterrupted however, it is generally interpreted that way. The interpretation is one of the many test criteria to be discussed. It would seem that a break in the test at the midpoint is not the same as having it cut short five minutes from the end because the vendor s boiler took an upset that was not related to the compressor test. The ibration during the shop test is normally specified as the API limit of 1.0 mils peak to peak, or the value from Equation 10.1, unfiltcred. whichever is lower. 

The undamped torsional natural frequencies of the complete train shall be at least 10 percent above or 10 percent below any possible (steady state) excitation frequency within the specified operating speed range (from minimum to maximum continuous speed). 

At any speed greater than the maximum eontinu-ous speed, up to and including the trip speed of the driver, the vibration shall not exceed 150 percent of the maximum value reeorded at the maximum continuous speed. 

Within the frequency range zero to 2.2 times maximum continuous speed, all the rotor s natural frequencies shall be calculated for the speed range 25-125 percent of rated, taking account of the following ... 

Pumps shall be capable of operating at least up to the maximum continuous speed. The maximum continuous speed shall be ... 

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