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Vol.59No.4April2009Compounding

Voices & Views

Controlling Deposit FormationUsing PAG Lubricantsby Martin Greaves, Ph.D.

In recent years there have been more

and more reports o equipment ailures

caused by sludge and varnish ormation

due to the degradation o hydrocarbon

oils1,2. I let untreated, varnish buildup

can result in severe problems or critical

components, including increased

bearing and gear wear and servo valve

ailure.

One o the undamental reasons

or this increase in ailures is that

equipment is being built to generate

more power. This in turn can increase

the stress on the lubricant and cause

excess rictional orces between

contacting suraces, higher uid

reservoir temperatures and, ultimately,

the thermal and oxidative degradation

o the lubricant.

When degradation happens,dangerous sludge and varnish deposits

can result. Controlling deposit

ormation in gear boxes or wind

turbines, gas turbines, hydraulic

circuits and, indeed, even automotive

engines remains a challenge when

using hydrocarbon-based lubricants,

especially when equipment is operating

at high temperatures or extended

periods.

More than 20 years ago, the

lubricants industry aced similar

challenges when using hydrocarbon

oils to lubricate rotary screw air

compressors. This was solved by

transitioning the lubricant technology

away rom hydrocarbon oils to synthetic

polyalkylene glycols (PAGs). Today there

are more than 100,000 air compressors

in service using PAG technology and the

problem o deposit ormation has

been essentially eliminated.

The image at right illustrates the

deposit control benefts o choosing

a PAG lubricant as compared to a

mineral oil product in a rotary screw

compressor. The same benefts can

also be achieved or reciprocating

and centriugal compressors using

PAGs.

Other applications also beneft

rom using PAGs to control deposits. In

the automotive industry, derivatives o

PAGs have been used as perormance

enhancing additives in top-tier gasoline

uels. They are extremely eective in

removing deposits on the uel injectors,

intake ports, intake valves, combustion

chambers and exhaust valves. Deposits

can lead to loss o power, knocking,poor drivability and the increased

emission o undesired hydrocarbon and

carbon monoxide gases.

How do PAGs minimize deposit

ormation and help maintain equipment

cleanliness and reliability?

The primary actor is the dierence

in thermal and oxidative breakdown

properties o hydrocarbon oils (mineral

oils and polyalphaolefns) compared to

PAGs. Simply put, when hydrocarbonoils degrade they orm high molecular

weight polar byproducts that become

insoluble in the hydrocarbon base

oil. Consequently, sludge develops

over time, and in some cases varnish

ormation occurs. This can have severe

consequences to equipment and, in

extreme cases, can lead to catastrophic

equipment ailure.

In contrast to hydrocarbon oils, PAGs

are extremely polar in nature. When

they degrade by oxidation, smaller

polar by-products are ormed, which

are soluble in the PAG base oil. The net

result is that sludge is almost eliminated

and equipment reliability enhanced.

In the power generation industry

there are several examples where

deposit ormation has become a major

issue. For example, gear box ailures in

wind turbines are notorious, and the

lubricant in these systems has never

been more challenged. Higher wind

turbine towers continue to be built, with

longer blades exerting huge loads on

the gears, drive shats and bearings. In

addition, wind arm operators expect

the gears to be operational or more

than 20 years.

Servicing wind turbines, especially

those which are oshore, incurs huge

costs. Advancing lubrication technology

beyond hydrocarbon oils by using PAGs

is being researched today as a potential

solution to reducing deposit ormation

and ultimately improving equipment

reliability. An additional beneft o

continued on page 26

Rotary screw compressors with PAG vs. mineral oil.

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26 CompoundingsApril2009Vol.59No.4

Voices & Views

a clean environment and reduce

problems such as servo valve sticking.

These new bio-based ormulations not

only provide better deposit control

compared to mineral oils and straight

vegetable oils, but they are also

biodegradable and derived rom a high

percentage o renewable eedstocks.

As well as their inherently good

deposit control eatures, PAGs also

provide many other outstanding

benefts. These include excellent

flm-orming potential and riction-

reducing properties, signifcantly higher

viscosity indices than hydrocarbon oils,

good low-temperature properties, and

attractive environmental profles. I

deposit control is a problem when using

lubricants made with hydrocarbon oils,the solution may lie in transitioning to a

PAG-based lubricant.

FOOTNOTES

1 Livingstone, Greg. Varnish: Looking Ahead.Turbomachinery International. Sept/Oct 2008.

2 Wagman, David. Lubrication: Industry leaders look

to x the varnish issue. Power Engineering. February

2008.

3 Profiet, Rob. Keep your hydraulic system clean o

varnish or pay the consequences. Lube Magazine.

No. 84, April 2008.

using PAGs or this application is their

excellent flm-orming properties

compared to hydrocarbon oils, resulting

in much lower riction orces in the

lubrication regime.

Hydrocarbon oils have been

successully used to lubricate gas

turbines or many decades, using

ormulations based on Group I mineral

oils. With the gradual phasing out

o Group I base oils, ormulators

have developed newer products

using Group II and III mineral oils.

But their introduction into the gas

turbine industry has been turbulent,

and deposit control remains a major

concern, particularly in North America,

where the newer ormulations have

been adopted aster.

Typically these ormulations contain

low levels o an additive package (e.g.

1-2 percent) containing anti-oxidants,

corrosion inhibitors, demulsifers and

anti-oams in the base oil. But the

more parafnic nature o the newer

base oils has resulted in the oxidative

degradation products being even lesssoluble in the base oil, accelerating

sludge and varnish ormation. Problems

associated with varnish ormation have

been reported, particularly with newer,

higher efciency turbines operating

at higher fring temperatures and

increased power output.

The economic consequences o a

turbine ailure in terms o equipment

damage and lost power production

are o paramount importance. With

global demand or energy expected to

double by 2030, reliable operation o

turbines will be critical going orward.

Higher quality lubricants, such as those

based on PAGs, could bring an end to

challenges o deposit ormation in gas

turbines similar to how history has

taught us about the benefts o using

PAGs in air compressors.

In hydraulic circuits, sticking o servo

valves due to varnish ormation is also

common3. This has been prevalent or

a number o years in systems operating

at high temperatures using hydrocarbon

oils, but it also appears to be occurring

with modern environmentally riendly

bio-based oils that use vegetable oil

eedstocks. The progression o hydraulic

equipment manuacturers striving

to produce units with higher power

output with smaller reservoirs has led

to elevated operating temperatures o

hydraulic uids. Consequently, the risk

o thermal and oxidative degradation

has increased, and thereore the risk o

varnish ormation has increased too.

Formulators o hydrocarbon-based

hydraulic uids have tried to addressthis challenge using new additive

technologies. But over time, additives

deplete, leaving the base oil vulnerable

to degradation. It seems logical that

lubricants using non-hydrocarbon-

based oils might solve these varnish

challenges. High perormance synthetic

hydraulic uids based on PAGs are now

able to solve these problems and keep

equipment running deposit ree with

signifcantly extended service intervals

and reduced equipment downtime.

New bio-based ormulation

technologies in which PAG base oils are

combined with vegetable base oils are

also emerging. These products utilize

the PAG to upgrade the vegetable

oils and provide a base uid that can

solubilize the oxidation by-products,

allowing equipment to operate in

Controlling Deposit Formation continued from page 25

Greaves is Technology

Leader for UCON

Fluids & Lubricants

with The Dow

Chemical Company.

He may be reached

at 979-238-3865 or

mrgreaves@dow.com

The economic consequences of a turbine

failure are of paramount importance.