free email newsletter - the maintenance war.


*** Topics from the front line. ***

1. Advice from the work place.
" Why you should trend the 'U' Values of your heat exchangers!
" Pump cavitation hammers your mechanical seals to pieces!
" Stress corrosion cracking of bolts depends on material selection.

2. Special Price for Pocket Maintenance Advisor

3. A Good Laugh!

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1. Advice from the Workplace

" Why you should trend the 'U' Values of your heat exchangers!
You can tell how well a heat exchanger is performing by measuring the inlet and outlet temperatures on both sides of the heat exchanger and calculating the theoretic heat transfer coefficient 'U' between the two fluids passing through the heat exchanger. The mathematics involved is simple for any mechanical or process engineer, and once formulated you can do it on a spreadsheet and graph it each day.

You want the 'U' value to be as high as possible. The 'U' value represents the lack of resistance to transferring heat between the two fluids. The more resistance there is to heat moving from one stream to the other, the greater the temperature difference needed to be to get the process temperature you want. This means unnecessary waste of heating or cooling.

I know that few operators get too excited if a heat exchanger is not running at prime efficiency. The only thing they want is to get the process temperature they need, and damn the expense.

But the expense is monstrous! It's a real profit killer! You can save your entire year's salary or wage with only a couple of degrees less temperature differential. Just look at this real example we discovered by accident the other day when we brought a new replacement heat exchanger on-line for the first time.

The new heat exchanger has clean copper tubes and cools the process down with chiller water. It is a small shell and tube heat exchanger about 2.5 meters long, 600 mm in diameter and with maybe 80 x 15 mm tubes running end to end. Compared to the old scaled-up heat exchanger we picked up a 5 oC reduction in differential temperature with the clean tubes. That represented a two-hour saving in batch processing time, which meant we could get one more batch out each week. This meant 52 more batches a year and another US$100,000 of sales.

That's what one small scaled-up heat exchanger was costing us. It's a lot of money, and anyone at your place that saves your company that sort of money each year, on the smallest of heat exchangers, is going to get a nice little bonus from the big boss sometime in the near future!

The way to get scale off a heat exchanger is either with mechanical cleaning or chemical cleaning. The cost of trending your heat exchanger performance is nothing. The cost of the cleaning the tubes will be significant, but the money you make, and the time you save with efficient heat exchangers more than pays for it many times over!

Don't hang about - start measuring temperature differentials, graph it and look for those massive savings (and a nice bonus!).  (TOP)

" Pump cavitation hammers your mechanical seals to pieces!
You know - it's amazing how much I don't know even after all these years as an engineer!

Why just the other day I learnt how mechanical seals get broken-up from cavitation. I guess I probably knew that cavitation could damage seal faces but I never thought about why that was so. The collapsing bubbles of gas cause pulsations that are like hammers hitting the back of the seal face on the process side. Each collapse creates a pressure surge that bangs into the seal. Bang it enough times and the seal cracks. Obvious really, but I never before thought about it.

The pressure pulses from cavitation bubble collapses send liquid spurting out at jet speeds. These pulses hit the back of the seal. Force is pressure x area. Big pressure pulses over the area of the seal mean a tremendous impacting force. Just think of it as a hammer bashing the back of the seal and you have got the right impression.

You want to quickly stop pumps cavitating, otherwise be prepared to change out a lot of mechanical seals. Some other options would be to use packed gland (stuffing box) pumps or maybe a sealess pump. The pressure pulses still happen, but at least there is no mechanical seal to get smashed-up (though you might lose an impeller or two).  (TOP)

" Stress corrosion cracking of bolts depends on material selection.
This tip is not one of mine. Rather it's from a friend - Richard Lang, who runs the Abonnel Engineering machine shop here in Perth. He puts out a quarterly flyer to his clients and always adds in a short, interesting article for his customers. This is a summary of part of an article. I've shortened it further due to space limitations, but the gist of it is complete. What I liked about it was that it made me think "How many people would ever realise water in bolt threads could lead to stress corrosion cracking?"

High tensile bolts, or bolts machined from high tensile steels, are brittle (That's how they become high tensile.). In fatigue, i.e. cyclic loading, situations or under heavily applied loads, any moisture on the bolt shank can lead to stress corrosion cracking. The water rusts the metal, rust pits the steel and the pit becomes a stress raiser. Finally the high stresses in the bolt from cyclic loading, or a high-applied load, snap the bolt. This can all happen in a few shifts.

Imagine what people would say if the job you did yesterday snapped again today! All because the bolts used were made of the wrong steel or of the wrong heat treatment. If water is present with fatigue loaded bolts, you need to know what metal your bolts are made from, how they are surface treated, and whether they are suitable for all corrosive situations in the application! Even wash-down water is enough to cause problems if the wrong bolts are used.

One option to solve the problem is to go for a more ductile bolt of larger load bearing cross-section diameter. Another is to keep the water away from the bolt shank, and away from under the bolt head and nut. But I doubt you can keep it water-free for the years of life required in most applications.

I would never have known that unless Richard told me! Well done Richard!

Special Price for Pocket Maintenance Advisor
Of all the publications Feed Forward publish, the Pocket Maintenance Advisor is the best seller. It sells so well that I had to order 200 from the printer during the month. This meant the price came right down compared to the 50 I normally get.

Because the printing price has now dropped I am selling them to newsletter subscribers at AUD$20 (AUD$30 on the web site). Just fill in the order form attached and fax it back to me. Fax is secure for credit card transactions.  (TOP)

 A good Laugh!

NINETEEN THINGS THAT IT TOOK ME 50 YEARS TO LEARN
By Dave Barry
1. Never, under any circumstances, take a sleeping pill and a laxative on the same night.
2.If you had to identify, in one word, the reason why the human race has not achieved, and never will achieve, its full potential, that word would be "meetings."
3.There is a very fine line between "hobby" and "mental illness."
4.People who want to share their religious views with you almost never want you to share yours with them.
5.And when God, who created the entire universe with all of its glories, decides to deliver a message to humanity, He WILL NOT use, as His messenger, a person on cable TV with a bad hairstyle.
6.You should not confuse your career with your life.
7.No matter what happens, somebody will find a way to take it too seriously.
8.When trouble arises and things look bad, there is always one individual who perceives a solution and is willing to take command. Very often, that individual is crazy.
9.Nobody cares if you can't dance well. Just get up and dance.
10.Never lick a steak knife.
11.Take out the fortune before you eat the cookie.
12.The most destructive force in the universe is gossip.
13.You will never find anybody who can give you a clear and compelling reason why we observe daylight savings time.
14.You should never say anything to a woman that even remotely suggests that you think she's pregnant unless you can see an actual baby emerging from her at that moment.
15.There comes a time when you should stop expecting other people to make a big deal about your birthday. That time is age eleven.
16.The one thing that unites all human beings, regardless of age, gender, religion, economic status or ethnic background, is that, deep down inside, we ALL believe that we are above average drivers.
17.The main accomplishment of almost all organized protests is to annoy people who are not in them.
18.A person who is nice to you, but rude to the waiter, is not a nice person. (This is very important. Pay attention. It never fails.)
19.Your friends love you anyway.  (TOP)

Short and sweet this month!

Best regards,

Mike Sondalini

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