I.Preface

Chuanxibei Gas Purification Plant of West-south Oil&Gas Branch Company in PetroChina locates in Jiangyou city Sichuan province. It takes natural gas from Jiangyou Zhongba gas field as raw material and makes purification. It is a key enterprise in Sichuan with daily purifying capacity 500000m³. Its main products includes sulfur, liquified petroleum and natural gas. The products have been sold to domestic and foreign markets. This purification plant belongs to typical petroleum chemical industry. There are kinds of equipments with high value in plant, so it requires highly about security and continuous operation.

There are two pumps 1# and 2# in purification plant. One works and the other is spare. Two motors started in direct on line. Big start current affects equipment lifetime as well as power grid. In desulfuration technics, temperature difference between daylight and night is big. Circulating flow should be adjusted correspondingly. In original constant speed operation, flow was adjusted by valve opening. By changing pipe resistance and increasing throttle loss adjusted flow. Much energy wasting resulted in high power cost. Valve adjustment not only brought inferior economic benefit but also increased worker labour. If worker did not adjust in time, it would cause too high or too low pressure, too big or too small flow. In order to reduce dusulfuration cost and heighten technics precision and efficiency, it was urgent to innovate with technology of variable frequency for 1# and 2# pump.

After long term of research, inspection and comparison, the plant decided to use HARSVERT-A series VFD.

II. Project introduction

1.There are five parts in desulfuration system: cooling tower, middle tank, circulating pump, flooding pump and desulfuration device. Circulating hot water from desulfuration device flows to middle tank through cooling tower. Most water would reuse via desulfuration. Excessive water spills from flooding pipe. Please look at process drawing in following:

2. Technics requirement

a. Temperature difference of input/output cooling tower is constant

Temperature difference is required to be constant(4℃<△t<8℃). If valve opening of circulating pump increases, flow increases, temperature difference of cooling water reduces. If flow reduces, temperature difference increases.

b. Minimum pressure control

Ensuring inlet pressure of desulfuration device (more than 0.45Mpa), VFD should save circulating flow as much as possible and find the lowest critical pressure.

3. Site state

VFD room is 7200mm in length and 3000 in width. (the space is too small to install 2 VFDs in parallel)

4. Instruments and meter

A pressure transmitter: single route output current 4～20mA, load capacity300Ω (two wire).

A temperature difference transmitter: single route output current 4～20mA, load capacity300Ω (four wire).

5. Parameters of motor and pump

III. Commissioning and Scheme

1. Space

Take account of limited space, we decide to put VFD and bypass cabinet separatedly. 1#, 2# VFDs  (3300×2574×1200 mm）install in parallel in VFD room and bypass cabinets install in pump site, which not only brings advantage to observe power delivering state but also solves problem of limited space.

2. Critical point

According to technics requirement, we get that pressure is sufficient condition of ensuring desulfuration production. Namely only when pressure reaches 0.46Mpa, the production can be ensured. If pressure of pump outlet is too low, it can not overcome potential energy of water and can not deliver circulating water to cooling tower. If the pressure is too high, output water of pump increases and efficient of cooling circulating water is low. So we decide to adopt control method of constant pressure close loop. Adjust given frequency of VFD and find the lowest critical point of pressure. This method can satisfy pressure demand and circulating flow demand, keep temperature difference of cooling tower in 4℃～8℃ stably. After repeated tests and demonstration, we get that when given frequency is 43Hz, pressure (0.51 Mpa) satisfies technics requirement, temperature difference is 4.92℃. So 43Hz is critical point.

Commissioning data:

3. Load capacity of transmitter

Because there is only one single route pressure transmitter of 300Ω in site, but input resistance of EM235 is 250Ω which is analogue input module of S7-200 PLC inside VFD. If analogue input circuits of 2 VFDs connect in series, input resistance of 2 EM235 is 500Ω. Single transmitter can not drive 2 pieces of EM235. So this method is infeasible. Therefore, we connect pressure transmitter to EM235 of single VFD, take advantage of module EM232 to output one route pressure of site. Make this output signal as analogue input of EM235 in the other VFD. Principal drawing is following:

In this method, no cost (hardware) added, by changing PLC programme makes EM232 of 1# VFD input a route pressure feedback signal instantly. It solves problem of insufficient capacity of pressure transmitter and realizes single pressure transmitter to drive 2 VFDs in close loop control.

IV. Energy saving calculation

Parameters of motor and pump:

In valve control mode, power consumption is product of flow Q and pressure H.

P_70%=315×0.75×（1.4-0.4×0.75×0.75）=277.6KW

In VFD control mode, power consumption is:

P_VFD=1.732×4880×23.76×0.96=192.8 kW

Energy saving rate is:（277.6-192.8）/277.6=30.5%

Yearly work time is 8000h, power price is 0.7yuan/kWh, cost saved of single pump is:

（277.6-192.8）×8000×0.70=475000yuan(USD 69750 )

V. Benefit of water saving

Because of special requirement of desulfuration technics, temperature difference of cooling tower decides circulating flow. Before VFD reform, in winter of big temperature difference, circulating flow demand was less. Excessive water was discharged from flooding pump, which caused much water wasted. After VFD reform, adjusting given frequency reduces circulating flow and ensures water temperature of desulfuration technics. Pump works in high efficient range and flooding loss is controlled.

VI. Concluding remarks

Practise proves that: HARSVERT-A06/040 VFD application in circulating pump of Chuanxibei Gas field is successful. Energy saving effect of VFD is obvious. Valve inlet/outlet opens fully, which reduces throttle loss. Soft start of motor prolongs motor lifetime. Inside PLC collects pressure data (4~20mA signal）from site, adjusts VFD output frequency automatically according to change of set value and actual value, implementing constant pressure water supply. VFD heightens automatization level of desulfuration technics and it has excellent utilization value.