Pipeline hydrotesting in winter temperatures.
Pipeline hydrotesting in winter temperatures.
(OP)
80% of pipeline is backfilled, remaining 20% is exposed. we use weighted values for start and end temps to give us overall DT that is used for allowable DP for the overall DT in the Shell Depp formula.
On all previous tests, as normal we see a pressure drop when temperature drops and calculate our DT as follows:
Start temp = (Tground*0.8)+(Texposed*0.2)
Start temp = (7.24*0.8)+(2.2*0.2)
Start temp = 6.232
End temp = (Tground*0.8)+(Texposed*0.2)
End temp = (7.18*0.8)+(1.6*0.2)
End temp = 6.064
DT = 0.168 Drop
The most recent test was carried out in very cold conditions and the relationship between P and T inversed on the exposed pipe. In general when the ambient got very cold so did the exposed pipe and the pressure went up. When ambient went up so did the exposed pipe and the pressure went down. Tground remains almost constant but behaves in the normal fashion (slight Tdrop for drop in pressure).
This left us concerned on how to calculate the DT and i question whether to use the same method as before. ie
Start temp = (Tground*0.8)+(Texposed*0.2)
Start temp = (7.24*0.8)+(0.73*0.2)
Start temp = 5.938
End temp = (Tground*0.8)+(Texposed*0.2)
End temp = (7.18*0.8)+(1.99*0.2)
End temp = 6.142
DT = 0.204 Gain
The effect of the Exposed pipe has caused the overall DT to be a gain, when the pressure has actually dropped (Pstart 155,465 Pend 155,380). This causes obvious problems.
Now as the inverse relationship is occuring would it be correct to say that as the exposed pipe temp increase should be used as a drop i.e.
Start temp = (Tground*0.8)+(Texposed*0.2)
Start temp = (7.24*0.8)+(0.73*0.2)
Start temp = 5.938
End temp = (Tground*0.8)+(Texposed*0.2)
End temp = (7.18*0.8)-(1.99*0.2)
End temp = 5.346
DT = 0.592 Drop
Which is preferable, but is it correct?
Thankyou in advance





RE: Pipeline hydrotesting in winter temperatures.
RE: Pipeline hydrotesting in winter temperatures.
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Pipeline hydrotesting in winter temperatures.
Were going to try and cover the exposed sections with rockwool/hay to stop them freezing. We are still unsure on exactly how to handle the previous test.
RE: Pipeline hydrotesting in winter temperatures.
I have no idea what a “Shell Depp formula” is. Please post the formula with units.
I assume that the temperatures listed are in Celsius. If this is so, then you are working in a very non linear area of temp-density relationship for water. Around 36 degF the density of water starts to decrease as the temperature drops. Above this temp the same is true. There is an inflection point around 36 degF in the curve. This is why most lakes do not freeze all the way to the bottom in cold climates (that is another topic that I will not expand on here). Your assumptions of using (Tground*0.8)+(Texposed*0.2) for an equivalent water temperature will only be accurate when your fluid density is linear with respect to temperature.
RE: Pipeline hydrotesting in winter temperatures.
We agree to cleverly call pipe testing "hydrostatic testing". That means put test pressures on pipes, but note the pressure for only limited amount of time, 2 hours maximum. Insulation is good, just to prevent temperature change during these two ours (between 1 and 3 PM is the best during the winter). Temperature measuring is good, just to make sure there are no temperature changes during this period.
This is good enough for reliable test results. If you are inclined to burst safety, you can hold pipe on pressure as long as you want having freeze protection implemented - just you don't need to officialy note pressure drop! If you hold pipes on pressure during say 12 hours, any leakage will show off as so big pressure drop that you will not need official formula to check with - common sense is reliable enough.
RE: Pipeline hydrotesting in winter temperatures.
RE: Pipeline hydrotesting in winter temperatures.
The point is not to stick to official noting of pressure over such period. Short period of 1-2 hours should be sufficiant for noting pressure differences while avoiding influence of temperature changes.