Read the Drain Bowl Before You Drain It

Why water sinks to the bowl

The bowl works because of density. A litre of mineral gear oil such as Mobilgear 600 XP weighs about 0.88 to 0.91 kg, and ExxonMobil's synthetic Mobil SHC Gear oils about 0.86 to 0.87 kg. ^{3 4} A litre of water is close to 1 kg, so when the oil is still, free water is heavier and sinks. ^{5 6}

Not all water in oil is free water. Oil holds some water dissolved in it, invisibly, up to a saturation limit set by the oil, its additives and its temperature. Above that limit, water shows as fine droplets that make the oil hazy, or as free water in its own layer. ^{5 6} As the oil cools, the limit falls, and dissolved water can come back out as fine haze after a stop. ^{5 7}

Separation also takes time. Stokes' law, which describes settling in a still liquid, says settling speed rises with the square of droplet size and falls as viscosity rises. ⁸ A droplet twice the diameter settles about four times faster, and cold, thick gear oil holds small droplets far longer than warm, thin oil. A bowl checked minutes after a stop can show much less than the same bowl after a night at rest.

Here is an illustrative case, not a reported one: a large gear drive on ISO VG 320 mineral gear oil, with a water-cooled oil cooler and a bowl at its drain. It runs all week, apart from the odd short stop, and stands idle every weekend.

Read it in the same state

Because settling depends on time and temperature, a reading can only be compared with another taken in the same machine state. Choose that state for each bowl, write it on the route card, for example “before start-up after an overnight stop”, and keep to it. A change in the bowl should then mean a change in the machine, not in the timing of the round. Luneta, which makes the bowl, recommends a daily check. ⁹ The frequency for each machine is the plant's decision.

The bowl shows only what reaches the port: if the port sits above the sump floor, some water can pool below it unseen.

For the gear drive, the route card now says: check daily at the start of the shift, note whether the drive was running or stopped, and note the date of the last drain. Each Monday pre-start reading is compared with the previous Monday's. That reading follows the longest rest of the week, so water and debris have had the most time to settle into the bowl.

What the bowl can show

Use the table as a starting point. The machine maker's instructions, the plant procedure and your oil analysis programme come first.

Haze in the oil

Haze has several possible causes. Luneta's guide for its Condition Monitoring Pod lists air, emulsified water and glycol for milky oil, and air, water, wax, additive dropout and mixed oils for slight haze. It says to confirm the cause by oil analysis. ¹⁰

If haze that used to clear on standing now stays, ask the laboratory for a water separability test as well, because an oil sheds water less readily as it oxidises or picks up contamination. ⁵

The usual test is ASTM D1401, or D2711 for heavy gear oils too thick to mix properly in the D1401 test. ^{11 12} Compare a D1401 result with the typical new-oil figure on the data sheet: new Mobilgear 600 XP takes 25 to 30 minutes at 82 °C to separate 37 mL of water, depending on grade. ³ A used oil that takes clearly longer is holding on to water.

Debris on the magnet

Magnet debris only means something against the same machine's history. Luneta's guide, written for the pod's magnet, grades debris from light fuzz through whiskers to heavy clumping. Light fuzz can be normal; heavy clumping should always go to the laboratory. ¹⁰ The bowl magnet keeps the iron and steel debris it catches until someone cleans it, so judge that debris, like the water layer, by how fast it builds up between photographs.

Some debris is expected after new parts are fitted and usually decreases once they have run in. ¹³ ASTM's guide to analysing wear particles under a microscope notes that many gearboxes produce some severe-wear particles throughout their service life, so judge each gearbox against its own history. ¹⁴ The magnet sits on the drain valve, so cleaning it means removing the valve: do that only with the sump drained, for example at an oil change. ¹⁵

A clean magnet says nothing about a bronze worm wheel or a white-metal bearing, because those metals are not magnetic. ¹⁶ Look for metallic grains on the floor of the bowl instead, and send them to the laboratory if they appear.

On these three fluids, an empty-looking bowl tells you very little about water. Follow the fluid supplier's method for checking it, and check the density of any other PAG with its supplier. Some detergent hydraulic oils of the HLPD type are also made to hold small amounts of water in suspension, so on them, too, an empty bowl does not prove the oil is dry. ⁵

Drain it with a record

Drain the bowl when it shows a layer or deposit, after you have read and photographed it. Do it under the plant's isolation and permit procedure, with the gloves and eye protection it specifies, and stand clear of the stream if the oil is warm. Use a clean, clear jar so you can see what comes out.

On the Luneta bowl, push the valve arms up and twist to open the valve, hold it open until clean oil has refilled the bowl, then close it. ^{9 20} Stay at the valve with a hand on it the whole time it is open. Turned fully, the valve locks open, which Luneta says is useful for draining the whole reservoir, so a valve left locked open can empty the gearbox. ^{9 15}

Then check the oil level, because oil leaves with the water, and top up through the plant's normal procedure. LubeServ's guide to how much oil to add when the level is low covers the method. Collect the drained oil and water for disposal through the site's waste procedure, not into a floor drain.

Do not send anything drained from the bowl as an oil sample. Luneta says the bowl's contents do not represent the oil at the gears, ⁹ and the US Bureau of Reclamation's lubrication manual says drain samples in general are likely to carry more contaminants than the oil that reaches the bearing. ²¹

A sample for trending belongs at a live-zone point, where the oil is moving and well mixed. That can be a supply line after the pump and before any filter, or a sampling tube that reaches into the middle of the sump. Draw it in the same machine state every time, ideally with the machine running. The bowl, by contrast, is best read at rest. ²¹ Where Clean Oil Gets Dirty explains how to keep routine samples comparable.

When water keeps returning

A single layer after a known event, such as a hose-down near an open fill cap, has a clear cause: record it, close the cap and check that the layer does not return at the next reading. A layer that keeps coming back is different.

Draining removes only the free water that has reached the bowl; water dissolved in the oil or held as fine haze stays behind, and cooling can push more of it out. ^{5 7} In thick gear oil that haze settles slowly, so a layer that is back at every reading usually means fresh water is getting in.

The usual routes in are:

• Humid air drawn in through vents and breathers as the machine warms and cools, then condensing inside (see The Breather Changed Colour)
• Washdown water, or steam cleaning that forces moisture in through the breather
• Worn or damaged seals, and fill points left open or poorly closed
• A leak inside a water-cooled oil cooler or heat exchanger
• Water carried in during top-ups, oil changes and sampling ^{5 6}

An internal cooler leak is hard to pin down because nothing drips outside the machine. Reliability Engine's article The Heat Exchanger Leak That Leaves No Puddle works through the same problem on data-centre liquid-cooling loops. Its method carries over to an oil cooler: read the pressure check above together with the oil level, the cooling-water make-up record, a sample from each side, and the maintenance and fill history.

Choose and fit a bowl

If a machine does not have a bowl yet, these details decide whether one will work. On a critical machine, agree any change to its drain with the machine's owner first.

• Fit it at a drain-level port. Luneta makes the bowl with 3/8, 1/2, 3/4, 1 and 1-1/2 inch NPT male threads, ^{9 22} and allows a bushing to change the thread. ²⁰ Check whether the port is NPT or BSP before ordering.
• Stay inside the ratings. Up to 93 °C and about 4.5 bar (65 psi), away from moving machine surfaces, shock, fluid surge and foot traffic. ⁹
• Match the size to the oil. Luneta does not recommend the 3/8 inch bowl with high-viscosity oil, because thick oil can seal over the top of the bowl and prevent proper flow through it. ²⁰ A bowl that never refreshes shows stale contents.
• Check compatibility. The bowl is made of Tritan with Viton seals and metal fittings, and Luneta lists cleaning solvents to avoid. ^{9 22} Luneta flags phosphate esters as possibly marginal for its pod, which uses similar materials, so confirm unusual fluids with the supplier. ²⁰
• Tighten it correctly. On the smaller sizes, tighten on the metal thread reducer, not the plastic hex. The 1-1/2 inch bowl screws in on its own moulded thread. Use a sealant suited to plastic threads rather than PTFE tape, and do not overtighten. ^{9 22}

A Condition Monitoring Pod complements the bowl. It sits at the oil-level port, where its sight glass shows level, colour and clarity while the machine runs, and its pilot tube draws a sample from inside the oil rather than from the bottom. It also carries a magnetic plug and a corrosion indicator. ^{10 23} On a critical gearbox the bowl gives a quick daily look at what settles, and the pod gives a proper sampling point.