Processing, April 2018

examined and evaluated by senior technical staff Also shortcomings in basic designs are usually discovered and mitigated in the detail design On the other hand in the areas of detail design installation and construction surprises are not uncommon Faulty or absent venting arrangements occur frequently For example if vessel vents are too close to the inlet nozzles feed materials might be vented unintentionally Gas locking may occur of pumps protected by non return valves without vents on the upstream side Errors are often made in gasphase balancing leading to ow and level di culties particularly with systems under vacuum or slightly positive pressure A pump set up with insufficient suction head to meet NPSHR has been reported during commissioning of many plants The situation may be worsened by partial suction restriction due to the plugging of a temporary strainer or similar condition The results in terms of cavitation could be problematic and damaging Lack of detailed assessments where a flow is split often leads to unsatisfactory results Examples are a sampling stream in parallel with the main flow and a level control maintained by recirculation as well as out ow with a control valve on one ow only In general such a problem can happen in any place where the ow is split When a control valve on a line is actuated the ow of both lines would be a ected and accurate control of these flows is not usually possible by only one valve Sometimes a dedicated control valve is needed on each line to properly control ow of each line In many operations such as di erent process units reactors separation units recovery systems liquidliquid separation liquid solid separation screening size reduction machinery mills etc and gas liquid separation the performance is usually worse than theoretically predicted or anticipated A wise design should assume imperfection and make contingency plans Extensive simulations as part of pre commissioning have proven to be worthwhile leading on occasion to some changes and provisions prior to commissioning But it is disappointing to learn at the startup that the equipment or system is operating far below theoretical expectations and there is no contingency plan and no safety margin factor e automatic control is another hot topic in the commissioning Whereas realistic simulation exercises attempt to test control performance as much as possible there have been severe limitations to do so in the absence of job fluids and normal operating conditions Commissioning plans should therefore be viable on the assumption that some control devices will not operate when needed and alternative means of operation and control should be included Di culties have been experienced in nonlinear control functions and control systems with large lags Systems involving large lags between action and detection have been the source of issues Controls based on detection of physical variables other than temperature pressure ow and level for instance those based on electrical or thermal conductivity radiation absorption e ects etc have shown difficulties and challenges These control systems have not been commonly used and there is a wide range of problems associated with them Complex LEAD FREE SOLENOID VALVES FOR DRINKING WATER APPLICATIONS CONTROL IT DONT JUST GO WITH THE FLOW programming controls can also cause trouble Sudden major leaks from piping or equipment after pressure testing have been reported in some occasions but such leaks are relatively uncommon if the piping and facilities are properly fabricated inspected and tested However other types of disfunctional pressurization and leaks have been common in the commissioning There have been dangerous incidents where uid from a high pressure circuit leaked into an ancillary low pressure circuit through a faulty item such as defective equipment tube or valve is resulted in release of a large quantity of pressurized material and even high pressure flammable material Careful attention should be given to all systems including low pressure auxiliary and ancillary systems and their potential interfaces and exposures to other process or high pressure circuits For instance there was a case that high pressure gas in a compressor pressurized the lubrication oil system because of a faulty internal part inside the machinery which caused extensive damages Vacuum leaks are a common problem during commissioning of vacuum systems particularly air leaks into a vacuum system ere have been many reported cases of air leaks into barometric legs the drain connection and piping connected to the condenser drain as a tail leg vulnerable to air leak Less obvious is the leakage of air through safety valves provided for relief of pressure operation in the same equipment The need for relocation of machinery an equipment can also slow things down during commissioning For instance there were positive displacement pumps that were located high in a structure that LEAD FREE VALVES As new lead reduction initiatives are being introduced Jeferson is commited to providing our customers with a Lead Free alternative for valves intended for potable water service The same great Jeferson products you know and trust now with optional Lead Free materials and approvals Visit our new website www jefersonvalves com Providing the industry with more choices to meet lead content restrictions in potable water equipment JeffersonAd2 indd 1 3 22 18 12 21 PM 52 Processing APRIL 2018