During my reading for the procedure of the pipeline hydrostatic testing , I observed that it is required to flush the pipeline before testing ,  the contrcator insisted to flush the pipeline after the testing and during the disinfection process for the pipeline , Is it ok to test the pipeline without flushing ??

you should flush it so particulates, rocks, debris dont act as a plug in whatever potential leak the pipe may have.  Chances are low that the debris will actually plug it, but it makes it impossible if it is flushed. 

I suspect many project specifications address in some manner general sequence and required operations, or at least refer to consensus standards that have been around quite a while such as AWWA C600 for Installation... (and also referring to many aspects of flushing, disinfection, and hydrotesting etc. operations) of ductile iron pipelines, as well as AWWA C651,  Disinfecting Water Mains.  I believe these standards in particular "make for good reading" for those involved with many aspects of water work.

AWWA C651 requires in an early section 4.4.3.2 entitled "Preliminary flushing", "Before the main is chlorinated, it shall be filled to eliminate air pockets and flushed to remove particulates. The flushing velocity in the main shall not be less than 2.5 ft/sec (0.76 m/sec) unless the purchaser determines that conditions do not permit the required flow to be discharged to waste."  There are later requirements for "Final flushing", that of course require that not long after successful disinfection is complete the heavily chlorinated water be expeditiously removed (flushed) from the main ..."until chlorine measurements show that the concentration in the water leaving the main is no higher than that generally prevailing in the distribution system or that is acceptable for domestic use."
    
While I guess it may not necessarily dictating sequence of all operations nor mean there is risk in your allowance (as I suspect there may be special cases that may make this difficult?), AWWA C600 also states in section 4.3.9 Flushing, "Foreign material left in the pipelines during installation often results in valve- or hydrant-seat leakage during pressure tests. The pipelines shall be kept clean during installation. Thorough flushing is recommended prior to a pressure test. Flushing should be accomplished by partially opening and closing valves(ball valve) and hydrants several times under expected line pressure, with flow velocities adequate to flush foreign material out of the valves and hydrants."  Thus at least some flushing before test is actually known to benefit the contractor in getting successful pressure tests!
 
In any a case, it is hoped that all parties to a water project, including the installer/contractor etc., respect the end purposes of a pipeline (not the least of which is delivered water quality) and keep same in mind in all operations well upfront. In this regard, AWWA C651, "Disinfecting Water Mains" also states "In either case, it is strongly recommended that pipe crews be aware of the need to maintain clean pipes and avoid contamination."

1. AWWA C600 does not place a specific value on the maximum length of pipeline to be tested, as I suspect practices have varied much from locale to locale and even very long lengths have been tested and performed well (and of course this is a consensus standard).  Nevertheless, some authorities and perhaps in some cases Contractors on their own, with perhaps very good reason(s) sometimes perform tests at rather frequent intervals of perhaps a only kilometer or two or less.  While much ductile iron pipe is very simple to install, an example of a good reason for short length tests, at least early in construction of some large projects. arguably might be (say e.g. at least with a crew unfamiliar with the specific type of pipe or joints etc.) that small problems of e.g. potentially repetitive nature such as errors or carelessness in assembly by a particular crew etc. are caught and corrected before they become much bigger problems.  After all, a long line [and maybe particularly one with few if any valves and a lot of elevation change] etc. represents a lot of water to evacuate and re-fill as opposed to a much shorter line.
2. There is likewise no minimum value for test pressure in C600, although I will note significant pressure can be developed in some cases/practices just filling (and fully evacuating air from) even more or less gravity lines for test.  Similarly to "1", for whatever reasons some authorities nevertheless require quite high pressure testing of even gravity water or sewer lines that arguably have no significant pressure developed in service!  While ductile iron and its most common joints are of course eminently suited for handling very high pressures, one disadvantage of very high pressure testing where it may arguably not really be necessary is that with some layouts/testing it might require quite formidable thrust blocks, test bulkheads (temporary and/or permanent etc.), or quantities of restrained joint pipes that arguably may not really necessary for any real service loading.  In such cases, I guess it could be argued there may be unnecessary expense borne ultimately by the Owner etc.
3. Again AWWA C600 does not require a "48 hour" (I guess sort of "soaking"?) period before tests can be run, although I bet many lines are exposed more or less full for at least some length of time while the Contractor prepares the line for test (going around to air release and other vaults to make sure air it fully out, and other final operations etc.)  While I guess there could be some value to getting a line to sort of stabilize in many respects, including any absorption of water into linings etc. (e.g. under working/filling pressure for at least a little while prior to test), I'm not sure e.g. that if a full line can hold required test pressure with no loss for the test duration much more quickly there is anything necessarily wrong with that!           

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