Nitrogen Purging

at a certified weigh station.

The company I work for (metal fab) requires this. The recycler must weigh each load and provide receipts (weight empty and full) from a third party weigh station.

make him cross his heart and promise

or pinky-swear ?

YES!

Any method to measure the quantity delivered that was not approved before delivery may not be accepted as valid, regardless of the accuracy of the measurement.

16,000 kg (35,200 lbs) plus the weight of the whole apparatus is a lot for a truck to delivery in a skid mounted system. That weight of the apparatus and the truck may have road weight problems.

When the empty container/truck is weighed at the beginning make note of the ancillary items on the unit and the truck. Also make sure the truck has a full tank(s) of diesel. Photos, a pencil and paper are a great way to document this.

When the container is full do the same thing. See that the list and the photos match. Make sure the diesel tanks are again full. Also use the same certified truck scale if possible. You don't want to pay for fuel if the gross weight is with a full tank and the tare weight is any lower. Same with the hoses, etc. You pay for the difference at the cost of the weight of nitrogen and also anything that was on the load and truck when the gross weight was taken. Nitrogen + "extras" + vehicle + full fuel = stated gross. Empty weight - "extras" - fuel used = stated tare weight. You would be paying for the "extras" slipped off/on and the fuel used at the rate of the cost of nitrogen/kg. Last and most important, make sure the scale is certified and has an honest reputation. More than one thumb has been pressed on the beam of a truck scale for the gross weight.

If there was 300lbs of "cheat" you are paying for 300lbs of Nitrogen you didn't get.

Good Luck, Old Salt

I believe i stated that in post #1.

For the completely honest system, yes. For a very common way for unscrupulous people it is definitely NOT. Honest net weight = gross - tare. Dishonest net = gross weight as it appears - tare - weight reductions made between weightings. This is much more prevalent than most people would guess. For those who often do this they have it down to a invisible science unnoticed by the honest people.

Example: weight heavy = starting:gross weight

dishonest light weight = 100 gal, 2ea almost full 55gal drums, of diesel removed from tanks (based on 2 ea 120 gal saddle tanks and 7.5 lbs/gal for diesel), = 825 lbs; hoses removes from truck between heavy and light weightings = 200 lbs; driver in truck during heavy weight and out at light weighting = 200 lbs. Total dishonest tare weight = true gross weight (1st weighting) - additional dishonest weight reductions = dishonest net weight (825 + 200 + 200) of 1,225 lbs lighter than actual.

1,225 lbs at 50.19 lbs/ft³ = difference of 24.4ft³ more than actual. Cost is then higher based on very frequent dishonesty. Customer pays for the dishonest additional net weight.

Although most people involved in commercial weighing are completely honest and would seek corrections and criminal prosecution to the unscrupulous weighting, the fact is that it still exist. Some of the characters involved can be the driver, the scale operator (certified), material handlers, accounts payable, accounting, management, and numerous other area of responsibilities.

We are the ones who pay the price at the end. It would be nice if every net weight (1wt - 2nd wt = net wt.) was honest at you stated, but in reality the extra steps must be taken to verify the accuracy of the net weight. As stated assuring honest weighting takes more than just weights.

Good Luck, Old Salt

Good points! We have been 'taken' in the past... in similar manner. It's real handy if you have your own weighbridge.

Weigh the delivery vehicle, prior to the delivery. Weigh the vehicle after delivery. Subtract the last weight from the first. This will equal the weight of product delivered. It doesn't matter what the contract says about delivery loses. You contracted for a specific weight of product. Pay for only what you receive.

Good luck in the contest.

When we filled liquid oxygen and liquid nitrogen carts from the bulk storage tank, we were instructed to fill them till it comes out the vent. At the end of the session we always had a pan with several gallons of nitrogen and oxygen (a very pretty blue colour). A significant volume was lost between evaporation for pressure build up, cooling any warm tanks and what went out the vent. If your delivery is by this method, the supplier usually accounts for the losses and you may get charged for it or they eat the cost (and you pay for it in the price of the delivery).

Which does bring up an important point. Your empty tank will be much warmer than the liquid, it will boil off a lot of liquid as it cools to liquid nitrogen temp.

For safety's sake, make sure your tank is equipped with pressure relief valves and rupture disks that will vent it should pressure become a problem. Also never close both ends of a pipe or hose with liquid nitrogen in it; if you do, you just made a bomb.

Drew K

Is this a new contractor to you?? See if you can get any references from him and check to see how honest he has been with other customers. That is, of course, unless he is already going to the measurements as others have posted. That's probably the first thing you should do is ask him the question directly-how does he know how much is in the tank? The contractor must already have something set up to measure the product as I am sure he doesn't want to over ship either and not get paid for it.

I have never used a skid tank before, but have used on site bulk tanks for years and delivery trucks are equipped with appropriate meters which are calibrated on a regular basis.

You are going to need to know the volume of your skid's tank, its pressure before adding nitrogen and your tank's pressure after off-loading of liquid nitrogen.

http://www.aqua-calc.com/calculate/volume-to-weight/substance/liquid-blank-nitrogen

Do the math on this on-line calculator before and after to get the accurate weight measurement. If you prefer, take your before pressure reading and figure the math prior to delivery to see what your minimum pressure should be after the delivery. Once you have determined this minimum pressure for your system, you should know whether or not you are getting a good deal or not.

The order is based on mass. Does pressure matter? That's a rhetorical question.

We get loads of Nitrogen into our plant all of the time. The vessel which receives the nitrogen is calibrated for a conversion from pressure in the system based on volume or mass. It can be converted to give an accurate reading based on mass or pressure. There is no need of weighing the truck before and after to get the correct conversion to mass. The person delivering the load of nitrogen gives a before pressure of the system and an after pressure of the system. It will stand up in court if there is any question of a "light" load.

At one chemical plant I was responsible for, my predecessor was insistent that all bulk deliveries be weighed for net weight. That was what he would only accept, from his office with the shiny seat of his pants. After he left we did an informal study with out supplier of bulk liq nitrogen, Air Products. They would do all the weights at the plant with a scale next door. End result was that the meters on their trucks were more accurate than the scale was. We were glad that he was demoted and transferred elsewhere.

His method of weights was particularly full of problems if the shipment was in an ISO container. If that container was not put into our bulk storage right away or was connected to the manufacturing system and wouldn't be empty for more than a few days it created other problems.

Good Luck, Old Salt