Dehydrating breather (silica gel breather)

01

Size the silica gel to the oil volume and breathing

Maier's transformer dehydrating breather type L to DIN 42562 consists of at least an upper part (B1) and an oil trap (OV); further intermediate parts (B2), each filled with 1.2 kg of silica gel, can be added. The listed sizes run from L1 with 1.2 kg to L5 with 6 kg of silica gel, and Maier states that the size to be used depends on the oil volume and the frequency of air exchange. [1]

MP Filtri's SAW 115 desiccant breather, listed for hydraulic systems, oil storage, tanks, transformers, renewable energy and gearboxes, holds 0.32, 0.56 or 0.84 kg of silica gel in three lengths (154, 205 and 256 mm high) with total weights of 0.60, 0.90 and 1.20 kg. [2]

Editorial calculation: the DIN 42562 sizes rise in 1.2 kg steps, so L5 holds five times the desiccant of L1 (6 ÷ 1.2), while the largest SAW 115 holds 0.84 kg, under the 1.2 kg of a single DIN 42562 section. [1] [2]

Give the supplier the oil volume, transformer or tank type and rating, expected temperature swings and breathing, required silica gel quantity or service interval, oil-trap requirement, connection thread or flange, materials (glass, acrylic, aluminium, stainless steel connection parts), corrosion and UV class, and ambient temperature range.

02

Check the oil trap, flow, filtration and replacement indication

In the Maier design, air drawn in as the transformer cools passes through the oil container, is cleaned of dirt particles and is dried by the silica gel; the oil trap prevents permanent contact between the silica gel and moist ambient air, and air expelled as the temperature rises flows back the opposite way. Cylinders and oil containers are glass or acrylic glass, and a corrosion-resistant version has an aluminium housing with C5-M powder coating and, Maier states, 40% less weight. [1]

The SAW 115 combines filtration of solid particles (3 µm absolute, inorganic microfibre) with desiccant drying, for air flow up to 453 l/min at 0.02 bar pressure drop, with optional check valves to extend desiccant life and operation from −25 °C to +90 °C. Its silica gel turns from orange to white when spent, at which point the breather must be replaced; MP Filtri states breathers should be changed at least once a year. [2]

Editorial note: colour indication differs between desiccants and makers, so confirm the colour change of the supplied gel and keep the viewing window visible from the inspection position.

03

Apply a utility's change-out rule, pipe size and moisture limits

The US Bureau of Reclamation's transformer maintenance manual (FIST 3-30) recommends adding a desiccant air dryer to free-breathing transformers and to free-breathing conservators, and installing one where an auxiliary-tank sealing system has none. For bladder or diaphragm conservators, it says to examine the air dryer periodically and change the desiccant when approximately one-third of the material changes colour. Its inspection checklist lists 'Air Drier Desiccant Proper Color' among the conservator checks. [3] The manual notes that a combination of oxygen and moisture accelerates deterioration of cellulose insulation, and that moisture decreases the dielectric strength of the oil and causes sludge. [3]

Reclamation warns that a vacuum forms in the transformer if the piping between air dryer and conservator, or the dryer's air intake, is too small or partly blocked, because the bladder cannot take in air fast enough as oil level falls with rapidly dropping temperature; the vacuum can form bubbles in the oil, trip Buchholz or bladder failure relays and pull air in around weak gaskets. It recommends a minimum of ¾- to 1-inch piping. Check that the breather's connection and internal passages are not the restriction. [3]

The manual explains why dry breathing matters: each time the moisture in a transformer doubles, insulation life is cut by one-half. It says plans for a dry-out should be made at 2% moisture by dry weight in the paper, and never to exceed 2.5% in the paper or 30% oil saturation without drying out. For service-aged transformers it treats up to 35 ppm water as acceptable below 69 kV, 25 ppm from 69 kV through 288 kV, and no more than 20 ppm above 288 kV. [3]

04

Size a disposable breather for gearboxes and hydraulic tanks

Des-Case publishes a sizing table for its Standard Series disposable desiccant breathers, which use silica gel with a polyester filter element rated 3μ absolute (β 3 ≥200), a −20°F to 200°F (−29°C to 93°C) range and multi-fit threaded connections. For each model it gives the silica gel quantity, water adsorption capacity, maximum air flow and the maximum gearbox or storage tank volume and hydraulic reservoir volume it suits; the hydraulic limit is lower than the gearbox limit for every model. [4]

The table also gives a maximum reservoir fluid flow at 1 psid, from 37 gpm for the DC-BB to 120 gpm for the DC-2 to DC-4, and lists a smaller maximum hydraulic reservoir than gearbox volume for every model [4]. Give the supplier the reservoir volume, whether it is a gearbox, storage tank or hydraulic reservoir, and the maximum pump or return flow, not only the tank size.

Transformer breathers are sized differently. Maier's DIN 42562 type L uses 1.2 kg of silica gel per stage, from L1 with 1.2 kg to L5 with 6 kg, sized by oil volume and breathing frequency [1], and the Bureau of Reclamation's FIST 3-30 gives change-out practice for transformer dryers [3]. MP Filtri's SAW 115 series is another desiccant air breather range [2]; ask any supplier for the silica gel mass, the water it can hold and the change indicator.

Editorial calculation: the largest Des-Case Standard Series model holds 913 g of silica gel, about three-quarters of one 1.2 kg Maier stage; its 365 ml adsorption capacity against 913 g of gel is about 0.4 ml of water per gram. [4] [1]

Values are from Des-Case's Standard Series desiccant breathers page; HydroGuard and other series were not tabulated here.

05

Gel container, oil trap, filter and check valves in a breather

Of these parts, the oil trap is the one Maier credits with keeping the silica gel out of permanent contact with moist ambient air between breaths. [1]

06

Gel mass, air flow and connection for a transformer or tank breather

Transformer and gearbox breathers are sized on different bases here: Maier by oil volume and breathing frequency, Des-Case by reservoir volume and fluid flow. [1] [4]

07

Send the oil, breathing and installation data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Equipment: [transformer or tank; rating; oil volume]
Breathing: [temperature swings; load cycle]
Desiccant: [silica gel mass; indicator colour; service interval]
Seal: [oil trap; check valves]
Flow: [l/min; filtration]
Connection: [thread or flange]
Environment: [temperature; corrosion; UV]
Volume: [quantity]

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