Compressors startup

Branch judiciously. References should not be used to make the writer s life easier. Sending the user to other procedures or documents may make writing the procedure easier, but it makes the procedure very cumbersome. Operators and mechanics often complain that the number of references in a procedure is so great that it makes the procedure impossible to use. For example, a startup procedure refers to a compressor startup procedure which in turn refers to a turbine startup procedure, and so on. If you are just using a step or two from another procedure, copy the step to the procedure you are writing. 

Compressor startup without cooling water flow... 

Fuel, oxidant, and water delivery Natural gas compressor, anode recycle blower/compressor, cathode recycle blower/compressor, startup water pump, valves... 

Modern nitric acid plants are designed for energy self-sufficiency during normal operation. Except for the startup phase, process heat generated will equal energy consumed by the compressors. Moreover, in many cases surplus energy can be exported in the form of steam, for example. 

The required suction pressure to the expander is not available until the oxidizer tower reaches design pressure. With two compressors in operation this takes approximately 15 min. Since the expander wheel is mounted on the compressor pinion, enough flow must be available at startup to prevent overheating. The calculated windage loss of the expander wheel (40 hp) requires about 16,000 Ib/lir of flow to prevent overheating. 

Startup flow to the expander is designed to be supplied from the discharge of the compressor through a throttle valve and cooler. A careful study of the characteristics of the compressor blow-off valve and the expander startup valve was performed by Solvay process engineers. Based on these calculations, the startup cooler and valve were sized to supply the minimum required flow to tlie expander wheel. 

Troyan, J. E., series Hints for Plant Startup, Chemical Enginnering, Part I—Troubleshooting New Processes, November 14, 1960, p. 223 Part II—Troubleshooting New Equipment, March 20, 1961, p. 147 Part III—Pumps, Compressors, and Agitators, May 1, 1961, p. 91. 

It would appear obvious for startup, and in some cases full-time operation. that a suction strainer or filter is mandatory. The reason for ilic strainer is to keep Junk and pipe scale out of the compressor. Fines from pipe scale and rust will make short work of the internal bore of a cylinder and are not all that good for the balance of the components. In some severe ca.ses, cylinders have been badly damaged in a matter of a few weeks. The strainer should be removable in service for cleaning, particularly when it is intended for permanent installation. Under all circumstances, provision must be made to monitor the condition of the strainer. Much frustration has been expended because a compressor overheated or lost capacity and no one knew if the strainer had fouled or blinded. 

An additional capacity control method is the unloader. This method can be used in conjunction with clearance pockets to extend the range of control to zero capacity. On double-acting cylinders, unloading the individual sides one at a time will provide a two-step unloading of the cylinder. On multicylinder arrangements, the cylinders can be unloaded one at a time providing as many steps as cylinders operating in parallel. The unloaders can also be used to totally unload the compressor, as is necessary for electric motor driver startup. 

Inlet startup screens have been recommended for other compressors covereii in the earlier chapters. If the point has not been made yet. n should be with the axial compressors. Considering that most of the cost of the compressor is in the hundreds of vulnerable blades just waiting to be hit by some foreign object, it should be obvious that some protection is needed until the piping has been proven to be clear and clean. 

Figure 7-2. Speed-load curve for a centrifugal compressor and motor. Startup difficulty could occur at the trouble zone where the curves are very close [7].

A pressure switch located so as to sense falling pressure at the earliest moment should be used to activate the standby pump. The switch should be so connected to the system as to permit testing the startup circuit without shutting down the compressor. Figure 8-8 shows four piping arrangements. The figure at b would be the recommended method. 

If casing limitations are fixed by user-supplied relief valves, this information should be conveyed to keep the vendor from rating the compressors on other data. Evaluations can be more of a problem if the same design basis isn t universal with all vendors. Startup and shutdown consideration influence various components, shaft end seals, seal system pressures, and even thrust bearings in some instances. The use of an alternate startup gas, or the desire to operate a gas compressor on air to aid in plant piping dryout should be covered. 

Ideally the compressor can be placed on the foundation, aligned, grouted, and piping connected. Shortly thereafter, the unit can be started. Most of the time there are lags between the various steps. User careles.s-ness here can delay ultimate startup and may mar what otherwise might have been a very successful installation. 

Each installation will have a series of startup checks that should be made prior to going into operation. These will vary for each compressor. A little extra time to record the overall compressor operation will he valuable if problems arise. 

Trips should normally be designed so that they operate if the power supply is lost. If this will cause a dangerous upset in plant operation, then an alarm should sound when power is lost. Trips should be tested at startup if they have been worked on during a shutdown. Particularly important trips, such as those on furnaces and compressors and high-oxygen concentration trips, should always be tested after a major shutdown. 

Most state laws and safe practice require a safety relief valve ahead of the first stop valve in every positive displacement compressed air system. It is set to release at 1.25 times the normal discharge pressure of the compressor or at the maximum working pressure of the system, whichever is lower. The relief valve piping system sometimes includes a manual vent valve and/or a bypass valve to the suction to facilitate startup and shutdown operations. Quick line sizing equations are (1) line connection, (i/1.75 (2) bypass, ii/4.5 (3) vent, dl63 and (4) relief valve port, cU9. 

The aeration requirements of the deep shaft (vertical shaft bioreactor using flotation technology) are provided by two, 100 hp rotary screw compressors rated at a pressure of 7 kg/m2 (100 psig). Dissolved oxygen (DO) levels of 4 mg/L are maintained in the head tank, and during the startup phase of the plant a DO meter measured a dissolved oxygen concentration of 25 mg/L at the shaft bottom. 

The major disadvantage of large plants is their vulnerability to large losses. In 1967 an explosion and tire in a Cities Service oil refinery at Lake Charles killed 7 employees and injured 14.6 The damage and business interruption costs exceeded 30,000,000. Usually the losses are not this large. However, in 1966 there were 20 fires in the chemical and petroleum industry, which caused damages in excess of 250,000.7 Even if there is no fire, the failure of a bearing on an ammonia compressor can cause the plant to shut down for a number of days two days for cool down, one day for repairs, two days for startup. The loss in sales from this interruption alone could exceed 50,000 per day, or a total of 250,000. ... 

Troyan13 suggests that all pumps, compressors, turbines, and other equipment should be checked internally before they are turned on. He notes that such an inspection of a large gas compressor revealed rust and scale from nearby pipes, a welding rod, and a small pipe wrench within the cylinder itself. Starting up the compressor before these items were removed could have done enough damage that whole sections would need replacement. This could have delayed the startup for weeks or even months. 

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