When working with HDPE pipes take note that Modulus of Elasticity varies significantly by the temperature and duration of loading. I've performed the stress analysis of two major water treatment plants and as Borzki mentioned, you need to expect large displacements in the system due to it's high expansion rate. The de-rating of HDPE with Temperature is very important as well. Especially for the pipework installed outdoor exposed to solar radiation (when empty), the surface temperature could rise up to 60+ deg C.
the expansion rate for a PE100 material is about 2.4mm/m/10degC. so for a 50m header pipe installed outdoor you should expect (50mx2.4mm/mx3)=360mm where 3 is (50deg C-20deg C)/10degC assuming pipe being installed @20deg C and reaches to 50deg C in the sun.
Here in Australia there are some good guidelines handling PE pipework. You may simply go to
www.PIPA.com.au for a broad range of technical information regarding such material. for example for derating of HDPE you may see this page (
https://www.pipa.com.au/sites/default/files/document/attachment/pop013.pdf)
In applying modulus of elasticity, since it changes a lot, you'd need to know where you need this parameter for. For pipe support and span calcs I used the long term about 20yrs and for thermal stress I used 1hr as I assumed the temperature changes is not as quick as 3min or as long as 5 hrs. You may find some values from a supplier in Australia in here:
http://www.vinidex.com.au/technical/pe-pressure-pipe/e-modulus/