9.6 Air Compressors, Conveyors, Hoists and Cranes, Service Piping & Standby Power Generation

The supporting equipment WPI lists by name

The Wastewater Treatment equipment outline closes with ancillary equipment: air compressors, conveyors, valves, hoists and cranes, pipes and fittings, and auxiliary power: generator. The Water Treatment outline lists air compressors, blowers, emergency systems, and power generation systems, and the collection outline lists chain pull hoists and overhead cranes and fans (forced air, air extraction). This section covers that supporting equipment.

Air compressors

Plant air does more work than most operators realize: it operates pneumatic valve actuators and instruments, supplies air scour and bubbler level systems, drives air-operated diaphragm pumps, powers tools, and provides air for blow-down and cleaning.

Operating essentials:

• Drain the receiver. Compressing air condenses water, which collects in the tank. Water in the receiver causes internal corrosion and carries into instrument air lines where it destroys positioners and freezes in cold weather. Automatic drains are installed and their operation is verified; manual drains are opened daily.
• Air dryers and filters. Instrument air requires a refrigerated or desiccant dryer to reach a dew point below the coldest ambient temperature the lines will see. In Colorado, an instrument air line running through an unheated gallery will plug with ice if the dryer fails.
• Safety relief valve. A receiver is a pressure vessel. The relief valve is tested, never blocked or adjusted upward, and the receiver is inspected per code.
• Inlet filter cleanliness directly affects capacity and energy use.
• Unloading and cycling. Compressors that cycle continuously are oversized or the system is leaking. Compressed air leaks are the most expensive utility leak in a plant, because the energy to make the air is many times the value of the air itself.
• Never use compressed air to clean clothing or skin. Air injected under the skin causes embolism.

Conveyors, hoists, and cranes

Conveyors move screenings, grit, and dewatered cake. Belt, screw (auger), and shaftless screw designs are common. Guarding at nip points is mandatory, and a conveyor is a machine that must be locked out before any clearing, adjustment, or guard removal — reaching into a jammed screw conveyor has cost operators limbs.

Hoists and cranes are listed by WPI in both the treatment and collection outlines because operators routinely lift submersible pumps, mixers, and screen assemblies. Requirements:

• Rated capacity marked on the hoist and never exceeded, and load ratings for slings and hardware known.
• Periodic inspection of hooks (for throat opening and twist), chains and wire rope (for broken wires, kinks, and corrosion), brakes, and limit switches, with records.
• Never work or walk under a suspended load, and use tag lines to control it.
• Portable davit and jib arms at lift stations must be installed in a base rated for the load, and the base itself inspected.
• Retrieval-rated equipment for confined space rescue is a different rating than material handling; a hoist used for non-entry rescue must be rated for personnel.

Valves and piping

Beyond the process valves covered elsewhere, plant service piping includes air, water, chemical, and sludge lines. Practical items:

• Label and color-code every line by contents and direction of flow so no one guesses.
• Sludge lines need cleanouts, generous radius bends, and provision for flushing; they plug.
• Chemical lines need double containment or containment routing where a leak would be hazardous.
• Expansion provisions on hot blower discharge piping.
• Freeze protection on any line that can sit stagnant in an unheated space.

Standby power

Auxiliary power is listed as its own equipment item, and for good reason: a wastewater plant or lift station without power spills, and a water plant without power loses pressure and its disinfection barrier.

Components: the engine-generator set, the fuel system, the automatic transfer switch (ATS), load banks or shed logic, and cooling and exhaust.

Automatic transfer switch operation is the concept most often tested. On loss of utility power the ATS senses the failure, waits a short confirmation delay, signals the generator to start, waits for the generator to reach rated voltage and frequency, then transfers the load. On restoration of utility power it waits a longer stabilization delay, transfers back, and runs the generator unloaded for a cool-down period before shutting it down. A properly interlocked transfer switch makes it physically impossible to connect the generator to the utility, which prevents backfeeding — energizing a supposedly dead utility line and killing a lineworker.

Exercise and testing. A generator that has never run under load is not standby power. Standard practice is a weekly no-load or brief run plus a monthly or quarterly test under real or simulated load long enough to reach operating temperature. Running a diesel repeatedly at no load causes wet stacking, the accumulation of unburned fuel in the exhaust, which fouls injectors and turbochargers.

Fuel management. Diesel degrades and grows microbial contamination at the fuel-water interface. Tanks are kept full to limit condensation, treated with biocide and stabilizer, tested periodically, and polished or turned over on a schedule. Day tanks and their level alarms are checked, and fuel supply for an extended outage — Colorado blizzards and wildfires have produced multi-day outages — is part of the emergency response plan.

Routine checks: coolant and oil level, block heater operation (critical in Colorado winters), battery condition and charger operation, belts and hoses, air intake, exhaust integrity and carbon monoxide risk in enclosed spaces, fuel level, and the run-hours log.