There are piping jigs which hold two pieces in precise alignment while they are being tacked. Any piping fabricator will have something to do the job.
Pipe is generally welded in accordance with ASME IX qualified procedures by qualified welders. The procedures control all variables including welding process and conditions, filler metal etc. The procedure is qualified for a thickness range, a material class (for instance, austenitic stainless steels are P8)
Radiography is usually required to assure the quality of piping butt welds. A company specializing in radiography and other non-destructive examination shoots the X-ray, develops the film and interprets the results. They are looking for flaws such as lack of fusion, porosity, cracks etc. There are criteria for the acceptance of welds for code piping in a piping code such as ASME B31.3.
If a qualified welder welds to the correct procedure, the required NDE is done and passed and a hydrotest is completed, the weld is assigned a joint efficiency of 1, meaning that it can be taken for design purposes as at least as strong as the unwelded material.
If you are jetwashing with a fluid not more hazardous than water, per B31.3 the service might be considered Category D- nonhazardous. One thing that this would imply is that you would not need to cap the end of the lance and hydrotest it prior to putting it in service, if the owner agrees. It is difficult to overpressure a pipe which is open at one end. Is it fitted with a spray nozzle of some sort, or some other flow restriction device? If so, you should probably hydrotest it to ensure joint integrity.