×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

how tight is too tight?
2

how tight is too tight?

how tight is too tight?

(OP)
I started a new job recently at a district energy centre.  And one of the parts of my job is construction supervisor for all the various energy units.  And part of that job is ensuring everything meets requirements based on various testing, notably pressure tests.

At my last job, in a different industry we would have to do pressure tests, but they weren't critical and if the system held pressure for a couple hours we were just gave them a pass.  No trending of pressure, etc.

But at this new job, we test to the consultant's required specs.  And I find most tests are technically failing, but I feel like the test results show the systems are good enough to a warrant a pass.  But regardless, they didn't meet the spec, the test failed and they have to redo it.  And of course I'm the one that gets to tell the contractors, with years of experience, to do it all over again because they dropped 1 or 2 psi outside the allowed range.

Does anyone else feel like specifications sometimes are a little too tight?  I feel like if the consultants were on the site looking at the systems, they'd give them a passing grade, but they're not, and it's me who is obligated to say that the test failed or take on the responsibility myself.

How do you guys handle that situation?  If I was in the contractors shoes I'd think my company is ridiculous for these tight specifications.

Maybe I'm just venting over my frustration with consultants who are sitting in an office 100 kms away (no offense to those who are consultants).

RE: how tight is too tight?

Quote:

And I find most tests are technically failing, but I feel like the test results show the systems are good enough to a warrant a pass.
Imagine repeating this in front of a jury.

RE: how tight is too tight?

The contractors were aware of the specs prior to testing (I would hope!). Maybe "the contractors, with years of experience" should know what it takes to meet the specs.

Standards exist for a reason. In this case, it is not your job to pick them, only to test them and report the results. If they wanted you to have "wiggle room" they would have put words like "professional judgement" in the requirements, not numerical pressure readings.

You wouldn't want a heart surgeon to have gotten a grade of "not technically passing, but close enough because they were being picky" and sent on his way into your heart.

-- MechEng2005

RE: how tight is too tight?

This is one of those non-gray areas.  It either passes or it doesn't.  You don't even have to quibble about it.

RE: how tight is too tight?

(OP)
Ya, you guys are right.

Just feel like it's tedious.  But you're right, and they're re-doing it on Monday.

Thanks for the input.

RE: how tight is too tight?

Fixed testing methods, with clear pass/fail criteria sound good to me.  It takes away any possibility for someone to put you under pressure to pass something that shouldn't be passed.

- Steve

RE: how tight is too tight?

Leakage at an unknown location is a bad thing:  indeed that's why you're doing the testing in the first place.

Leakage at a known location might give sufficient reason to not require repeating a test.  Leakage from a valve packing is such an example.  Do you de-pressurize, fix the valve packing and then repeat the test, or do you pass it because you know where the leak was?  That's a judgment call, and unfortunately it's your client's judgment rather than yours.

Yes, it's possible for a test criterion to be set too stringently by an ignorant client.  Sometimes this requires that the system be broken into smaller pieces to permit testing, leaving the connections BETWEEN these pieces untested or incompletely tested.  That is not in the client's interest.

It's also possible to do a hydrotest poorly enough, i.e. by not ensuring that the equipment is 100% full of water without any trapped air, for a test to pass despite significant leakage.   

RE: how tight is too tight?

(OP)
I just used the pressure the test as an example.  We have other tests where I feel they're too stringent.

But as someone said, the test requirements are in the specs when the contractors bid.  They should be aware and do the test in a manner they should pass.  

I feel like their work is good enough to pass, maybe they just need to do conduct the test differently...

RE: how tight is too tight?

have you done a risk assessment in order to determine that "the systems are good enough to a warrant a pass"? Did the design engineer do it? It is possible that assessment might indicate undue caution regarding possible leakage, but how would you know otherwise?

RE: how tight is too tight?

In the test world, the spec is the spec period.  You can put in tolerances like 5% below and 20% higher or which way would give you more margin for the design you give higher percentage.  However, one can ask for a waiver from the customer if the customer wants to take the risk and accept the data.  But, when something fails later down the line, it will bite you in the butt and down grade your reputation.  One can rationalize whatever they want to be right, but keeping to the spec will keep one honest (and sleep at night).

Tobalcane
"If you avoid failure, you also avoid success."
"Luck is where preparation meets opportunity"  

RE: how tight is too tight?

Quote:

the test failed and they have to redo it.

So no corrective action?  Just repeat the test and hope it passes the second time?

And if it doesn't pass the second time just keep testing until it passes?

Most likely you have pass/fail criteria that are based on "design nominal" values, with some arbitrarily assigned range tack on.

Properly  chosen pass/fail criteria need to take into account process variability and instrumentation error.

RE: how tight is too tight?

Read "TheTick"'s post again.  If it doesn't sink in, read it again.  It's the world I live in...you don't want to be there as a defendant.

Pass/Fail...really simple. No interpretation required.

RE: how tight is too tight?

I think the converse is true, i.e., the tests are not "too tight," the contractors are "too loose," and are hoping to slide by because they cheated on the construction, somewhere, to save a buck or two.  THAT's what typically happens.  Yes, contractors do know what's required, most of the time, and what's required is often too expensive and competitive bidding forces everyone into a gigantic gray lie, i.e., that they can meet all the requirements ata fraction of what it really costs.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize

RE: how tight is too tight?

(OP)
I know you guys are right.  It is nice to get some justification for making them redo the test.

But, if the test fails again, they'll have to do it a third time, isolating the suspective cause.  And if it turns out that after isolating the valve the system passes, we'll probably just give them a pass on the whole system since it is a non critical leak.  

I'd say it was pointless, but I think there is something to say about holding them accountable and making sure they know that we will not let just anything pass.

It's still tough sometimes when you feel like you're splitting hairs.  But I do not want to be repeating to a jury that it was close to passing....

RE: how tight is too tight?

Showing my ingnorance here.  Is the temperature monitored during a test?  Wouldn't it be reasonable to have pressure change with changing temperature?

RE: how tight is too tight?

(OP)
Temperature is noted but not monitored.  It was one of a series of tests, in the same facility, and none of the other ones showed any significant change in pressure.  The pressure of the failed test varied noticably more than the others.



 

RE: how tight is too tight?

(OP)
I should have said temperature is noted but not recorded.  It probably should become part of the spec... I'm not going to get into why it hasn't.  That is another thread.

RE: how tight is too tight?

It's difficult to write specs to that level of detail.  If we did, we'd probably never get anyone to bid, since there would be zero wiggle room, and there might never be anyone who could claim compliance, since there's rarely anyway to verify that ALL performance can actually be met under ALL conditions, or any combination thereof.  We'd spend a year testing some silly little widget, and not really get any value-added.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize

RE: how tight is too tight?

Posting here after reading the responses and I am in total ignorance of what precisely "the spec" states.  However...  I come from the manufacturing engineering world where everything involves rigorous testing to prove / disprove quality.  At least, it should.  One of the most common "failures of process" I have seen over and over again is having unreasonable faith in your test method and test devices to generate useful data.  Are you absolutely certain that the pass/fail data you are generating is accurate?  Gages and other data-producing devices should be appropriate to the task, appropriate to the data range, calibrated by 3rd parties, and run through some flavor of a gage repeatability & reproducibility test to prove their ability to produce good, useful data under whatever test conditions are required.  Just a word of skeptical caution to make sure you are not generating false negatives or false positives, and causing a lot of unnecessary trouble.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering
www.bluetechnik.com

RE: how tight is too tight?

"just give them a pass on the whole system since it is a non critical leak"  If a leak were a non-critical, then it shouldn't be in a critical test path.  Either that, or there should be a looser requirement for the non-critical areas.  The fact of the matter is that the supplier proposed, and the client ostensibly accepted, the test procedure as written, hence the ongoing testing.

While retesting without correction is often done, and the next time the test is run, the article often passes.  This is usually an indication of a unreliable test, and raises the issue of whether a critical leak actually exists, but just was caught the testing.  That would be a bad deal all around.

TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize

RE: how tight is too tight?

(OP)
Well I feel like I should let everyone know that I had the crew redo the test.  And it passed.  Pressure held constant the entire length of the test.

I agree with many of the comments made.  I feel this spec is somewhat flawed but at the same time the crew approached it in a flawed manner as well.  this second time, they were more careful with conditions (temperature variations) and the results were better.  Too bad they didn't take those precautions the first time.

Anyway, I hope the next time they're working on our equipment they keep in mind the fact I will only pass a test that meets the passing criteria and they'll do it properly the first time.

I appreciate the comments though.  I like getting input from other engineers who have been in similar situations.

Cheers.

RE: how tight is too tight?

remember too, some industries (mine for example) are notorious for over-specifying and sometimes without justification.

i remember a job awhile ago when we were testing a system (a whole airplane's load of systems).  we took the prime's spec's and determined we needed a really special ground cart to create the conditions required by the specs.  when this was presented back to the prime the reaction was "what, are we building a plane that can't use a standard Hobart ground cart?" "no, we need this to cover the spec requirements." "well that's just dumb ... we never had to do this before (when we tested ourselves)"

RE: how tight is too tight?

From a guy who works on both design & post-installation issues, it's surprising how many times the specs do NOT pertain to the operation.

Your job is definitely NOT to turn a blind eye and pass things.  If you feel the spec is too tight, pass your comments up/across the chain, with all your reasoning, and see what your feedback does.

You never know, you may have someone thank you.  Odds are though, you'll be politely ignored.

RE: how tight is too tight?

My earliest engineering work had to do with justifying small mfg defects to the customer. These were rocket cases with small weld offsets in the skirts. My analysis showed acceptability, but the customer wanted to run a test. They set up an axial test at Huntsville, which passed without undue distress. The test justified my analysis, but the customer was not thrilled to be coaxed by the mfr to accept less than specified details.

In another company I helped introduce fracture mechanics analysis for structural welds with defects. That got the attention of a lot of people in USAF. There is a place for debating the means of accepting less than specified results in mfg.

RE: how tight is too tight?

I have seen specs put on things for extremely dubious reasons. As I am a a more industrial, hands-on engineer, this can be a large problem since, as you all know, the tighter the spec, the tougher it is to create product within spec. One of my jobs is to intentionally create parts out of spec (purely for experimental purposes, of course) and then test them in ways that best realistically simulate actual use conditions to see if the high specs are justified.

It's not that I'm combattative with the design team. It's that I have the luxury of being able to relatively quickly test things like this when the design team never comes within 50 miles of the parts. We are able to provide them with data about how the parts hold up when certain parameters are loosened. And sometimes things fail dramatically, and sometimes they fail undramatically, and sometimes they're just worringly enough out of spec to make me not want to question it. But sometimes there's no realistic change there at all.

None of this whatsoever says that you can justify ignoring or getting wiggle room specs. Follow the specs vigorously while they're the specs. But I think it's healthy to question them in ways that are valid both from a design as well as a use perspective. You might find some "well, it's always been like that" type situations that can be worked around to the benefit of all.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources